Merge branch 'users/chapuni/cov/single/base' into users/chapuni/cov/single/switchusers/chapuni/cov/single/switch

194 files changed, 1898 insertions, 1101 deletions

diff --git a/libcxx/.clang-tidy b/libcxx/.clang-tidy
index f986e2100ca6..ebbfab037926 100644
--- a/libcxx/.clang-tidy
+++ b/libcxx/.clang-tidy
@@ -5,6 +5,8 @@ Checks: >
   bugprone-stringview-nullptr,
   bugprone-use-after-move,
 
+  libcpp-*,
+
   llvm-include-order,
   llvm-namespace-comment,
 
diff --git a/libcxx/docs/Hardening.rst b/libcxx/docs/Hardening.rst
index 4002f40e1dad..d399b94d27c1 100644
--- a/libcxx/docs/Hardening.rst
+++ b/libcxx/docs/Hardening.rst
@@ -311,7 +311,10 @@ ABI configuration.
 ABI options
 -----------
 
-Vendors can use the following ABI options to enable additional hardening checks:
+Vendors can use some ABI options at CMake configuration time (when building libc++
+itself) to enable additional hardening checks. This is done by passing these
+macros as ``-DLIBCXX_ABI_DEFINES="_LIBCPP_ABI_FOO;_LIBCPP_ABI_BAR;etc"`` at
+CMake configuration time. The available options are:
 
 - ``_LIBCPP_ABI_BOUNDED_ITERATORS`` -- changes the iterator type of select
   containers (see below) to a bounded iterator that keeps track of whether it's
@@ -341,7 +344,7 @@ Vendors can use the following ABI options to enable additional hardening checks:
 
   ABI impact: changes the iterator type of ``vector`` (except ``vector<bool>``).
 
-- ``_LIBCPP_ABI_BOUNDED_UNIQUE_PTR``` -- tracks the bounds of the array stored inside
+- ``_LIBCPP_ABI_BOUNDED_UNIQUE_PTR`` -- tracks the bounds of the array stored inside
   a ``std::unique_ptr<T[]>``, allowing it to trap when accessed out-of-bounds. This
   requires the ``std::unique_ptr`` to be created using an API like ``std::make_unique``
   or ``std::make_unique_for_overwrite``, otherwise the bounds information is not available
@@ -407,7 +410,7 @@ Hardened containers status
       - ✅
       - ❌
     * - ``forward_list``
-      - ❌
+      - ✅
       - ❌
     * - ``deque``
       - ✅
diff --git a/libcxx/docs/ReleaseNotes/20.rst b/libcxx/docs/ReleaseNotes/20.rst
index c8a07fb8b733..ecfbaa5b7a37 100644
--- a/libcxx/docs/ReleaseNotes/20.rst
+++ b/libcxx/docs/ReleaseNotes/20.rst
@@ -73,6 +73,39 @@ Improvements and New Features
   optimized, resulting in a performance improvement of up to 2x for trivial element types (e.g., `std::vector<int>`),
   and up to 3.4x for non-trivial element types (e.g., `std::vector<std::vector<int>>`).
 
+- On Windows, ``<system_error>``'s ``std::system_category`` is now distinct from ``std::generic_category``. The behavior
+  on other operating systems is unchanged.
+
+  On Windows -- unlike on Unix systems -- the libc and system APIs use distinct error codes. The libc functions return
+  ``errno.h`` error codes via the ``errno`` global, while Win32 API functions return ``winerror.h`` error codes via
+  ``GetLastError()``.
+
+  The C++ standard's ``std::error_code`` and ``std::error_category`` functionality was designed to support multiple
+  error domains, precisely in order to handle situations such as this. However, libc++ formerly treated
+  ``generic_category()`` and ``system_category()`` as equivalent, even on Windows. It now implements the intended split,
+  where ``system_category`` represents native ``winerror.h`` error codes, and ``generic_category`` represents libc error
+  codes (and, equivalently, ``std::errc::*`` errors).
+
+  This change enables code like ``std::error_code(GetLastError(), std::system_category()) ==
+  std::errc::invalid_argument`` to function as desired: constructing an ``error_code`` with the Windows error number in
+  the "system" category, and then mapping it to a generic code with ``error_condition``, for comparison with the
+  ``std::errc`` constant.
+
+  This is an incompatible change: ``std::error_code(ENOSYS, std::system_category()) ==
+  std::errc::function_not_supported`` would formerly have returned true, but now returns false on Windows. Code
+  providing a number from the ``errno.h`` domain should be migrated to construct a ``generic_category`` error_code,
+  instead. (E.g., use ``std::error_code(ENOSYS, std::generic_category())``). The new behavior matches MSVC.
+
+- On Windows, the ``std::filesystem`` library now returns the Win32 ``system_category`` error codes, where it's feasible
+  to do so. This allows interrogation and reporting of the original error code, which is useful if multiple Windows
+  errors map to a single generic error (such as with ``std::errc::no_such_file_or_directory``).
+
+  This is also a slightly-incompatible API change: code inspecting the raw integer value from the returned error_code
+  expecting an integer from ``generic_category`` (e.g. ``err.value() == ENOTDIR``) will not work as desired. Instead,
+  such code should use the comparison operators which implicitly handle eror mappings, ``err ==
+  std::errc::not_a_directory``, or use ``err.default_error_condition()`` to map to an ``error_condition``, and then test
+  its ``value()`` and ``category()``.
+
 Deprecations and Removals
 -------------------------
 
diff --git a/libcxx/docs/TestingLibcxx.rst b/libcxx/docs/TestingLibcxx.rst
index cf092fabd046..e98b96bfb478 100644
--- a/libcxx/docs/TestingLibcxx.rst
+++ b/libcxx/docs/TestingLibcxx.rst
@@ -459,6 +459,29 @@ we only want to make sure they don't rot. Do not rely on the results of benchmar
 run through ``check-cxx`` for anything, instead run the benchmarks manually using
 the instructions for running individual tests.
 
+If you want to compare the results of different benchmark runs, we recommend using the
+``libcxx-compare-benchmarks`` helper tool. First, configure CMake in a build directory
+and run the benchmark:
+
+.. code-block:: bash
+
+  $ cmake -S runtimes -B <build1> [...]
+  $ libcxx/utils/libcxx-lit <build1> libcxx/test/benchmarks/string.bench.cpp --param optimization=speed
+
+Then, do the same for the second configuration you want to test. Use a different build
+directory for that configuration:
+
+.. code-block:: bash
+
+  $ cmake -S runtimes -B <build2> [...]
+  $ libcxx/utils/libcxx-lit <build2> libcxx/test/benchmarks/string.bench.cpp --param optimization=speed
+
+Finally, use ``libcxx-compare-benchmarks`` to compare both:
+
+.. code-block:: bash
+
+  $ libcxx/utils/libcxx-compare-benchmarks <build1> <build2> libcxx/test/benchmarks/string.bench.cpp
+
 .. _`Google Benchmark`: https://github.com/google/benchmark
 
 .. _testing-hardening-assertions:
diff --git a/libcxx/include/__algorithm/comp_ref_type.h b/libcxx/include/__algorithm/comp_ref_type.h
index c367fbb91ac2..6a9d5cef2671 100644
--- a/libcxx/include/__algorithm/comp_ref_type.h
+++ b/libcxx/include/__algorithm/comp_ref_type.h
@@ -56,10 +56,10 @@ struct __debug_less {
 // Pass the comparator by lvalue reference. Or in the debug mode, using a debugging wrapper that stores a reference.
 #if _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
 template <class _Comp>
-using __comp_ref_type = __debug_less<_Comp>;
+using __comp_ref_type _LIBCPP_NODEBUG = __debug_less<_Comp>;
 #else
 template <class _Comp>
-using __comp_ref_type = _Comp&;
+using __comp_ref_type _LIBCPP_NODEBUG = _Comp&;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/__algorithm/copy.h b/libcxx/include/__algorithm/copy.h
index 4f30b2050abb..962aa90059d5 100644
--- a/libcxx/include/__algorithm/copy.h
+++ b/libcxx/include/__algorithm/copy.h
@@ -47,7 +47,7 @@ struct __copy_impl {
 
   template <class _InIter, class _OutIter>
   struct _CopySegment {
-    using _Traits = __segmented_iterator_traits<_InIter>;
+    using _Traits _LIBCPP_NODEBUG = __segmented_iterator_traits<_InIter>;
 
     _OutIter& __result_;
 
diff --git a/libcxx/include/__algorithm/iterator_operations.h b/libcxx/include/__algorithm/iterator_operations.h
index 6cdb0aec9b2d..e5c89c1e67e3 100644
--- a/libcxx/include/__algorithm/iterator_operations.h
+++ b/libcxx/include/__algorithm/iterator_operations.h
@@ -48,13 +48,13 @@ struct _RangeAlgPolicy {};
 template <>
 struct _IterOps<_RangeAlgPolicy> {
   template <class _Iter>
-  using __value_type = iter_value_t<_Iter>;
+  using __value_type _LIBCPP_NODEBUG = iter_value_t<_Iter>;
 
   template <class _Iter>
-  using __iterator_category = ranges::__iterator_concept<_Iter>;
+  using __iterator_category _LIBCPP_NODEBUG = ranges::__iterator_concept<_Iter>;
 
   template <class _Iter>
-  using __difference_type = iter_difference_t<_Iter>;
+  using __difference_type _LIBCPP_NODEBUG = iter_difference_t<_Iter>;
 
   static constexpr auto advance      = ranges::advance;
   static constexpr auto distance     = ranges::distance;
@@ -72,13 +72,13 @@ struct _ClassicAlgPolicy {};
 template <>
 struct _IterOps<_ClassicAlgPolicy> {
   template <class _Iter>
-  using __value_type = typename iterator_traits<_Iter>::value_type;
+  using __value_type _LIBCPP_NODEBUG = typename iterator_traits<_Iter>::value_type;
 
   template <class _Iter>
-  using __iterator_category = typename iterator_traits<_Iter>::iterator_category;
+  using __iterator_category _LIBCPP_NODEBUG = typename iterator_traits<_Iter>::iterator_category;
 
   template <class _Iter>
-  using __difference_type = typename iterator_traits<_Iter>::difference_type;
+  using __difference_type _LIBCPP_NODEBUG = typename iterator_traits<_Iter>::difference_type;
 
   // advance
   template <class _Iter, class _Distance>
@@ -94,10 +94,10 @@ struct _IterOps<_ClassicAlgPolicy> {
   }
 
   template <class _Iter>
-  using __deref_t = decltype(*std::declval<_Iter&>());
+  using __deref_t _LIBCPP_NODEBUG = decltype(*std::declval<_Iter&>());
 
   template <class _Iter>
-  using __move_t = decltype(std::move(*std::declval<_Iter&>()));
+  using __move_t _LIBCPP_NODEBUG = decltype(std::move(*std::declval<_Iter&>()));
 
   template <class _Iter>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 static void __validate_iter_reference() {
@@ -217,7 +217,7 @@ private:
 };
 
 template <class _AlgPolicy, class _Iter>
-using __policy_iter_diff_t = typename _IterOps<_AlgPolicy>::template __difference_type<_Iter>;
+using __policy_iter_diff_t _LIBCPP_NODEBUG = typename _IterOps<_AlgPolicy>::template __difference_type<_Iter>;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__algorithm/move.h b/libcxx/include/__algorithm/move.h
index 005099dcac06..6f3b0eb5d292 100644
--- a/libcxx/include/__algorithm/move.h
+++ b/libcxx/include/__algorithm/move.h
@@ -50,7 +50,7 @@ struct __move_impl {
 
   template <class _InIter, class _OutIter>
   struct _MoveSegment {
-    using _Traits = __segmented_iterator_traits<_InIter>;
+    using _Traits _LIBCPP_NODEBUG = __segmented_iterator_traits<_InIter>;
 
     _OutIter& __result_;
 
diff --git a/libcxx/include/__algorithm/ranges_iterator_concept.h b/libcxx/include/__algorithm/ranges_iterator_concept.h
index 2af891d3af00..58790e95aa80 100644
--- a/libcxx/include/__algorithm/ranges_iterator_concept.h
+++ b/libcxx/include/__algorithm/ranges_iterator_concept.h
@@ -44,7 +44,7 @@ consteval auto __get_iterator_concept() {
 }
 
 template <class _Iter>
-using __iterator_concept = decltype(__get_iterator_concept<_Iter>());
+using __iterator_concept _LIBCPP_NODEBUG = decltype(__get_iterator_concept<_Iter>());
 
 } // namespace ranges
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/__algorithm/ranges_unique_copy.h b/libcxx/include/__algorithm/ranges_unique_copy.h
index 3b4a64e94ca1..ee7f0a0187b7 100644
--- a/libcxx/include/__algorithm/ranges_unique_copy.h
+++ b/libcxx/include/__algorithm/ranges_unique_copy.h
@@ -60,7 +60,7 @@ struct __unique_copy {
   }
 
   template <class _InIter, class _OutIter>
-  using __algo_tag_t = decltype(__get_algo_tag<_InIter, _OutIter>());
+  using __algo_tag_t _LIBCPP_NODEBUG = decltype(__get_algo_tag<_InIter, _OutIter>());
 
   template <input_iterator _InIter,
             sentinel_for<_InIter> _Sent,
diff --git a/libcxx/include/__algorithm/simd_utils.h b/libcxx/include/__algorithm/simd_utils.h
index 4e3e4f2b9404..4e03723a3285 100644
--- a/libcxx/include/__algorithm/simd_utils.h
+++ b/libcxx/include/__algorithm/simd_utils.h
@@ -70,7 +70,7 @@ struct __get_as_integer_type_impl<8> {
 };
 
 template <class _Tp>
-using __get_as_integer_type_t = typename __get_as_integer_type_impl<sizeof(_Tp)>::type;
+using __get_as_integer_type_t _LIBCPP_NODEBUG = typename __get_as_integer_type_impl<sizeof(_Tp)>::type;
 
 // This isn't specialized for 64 byte vectors on purpose. They have the potential to significantly reduce performance
 // in mixed simd/non-simd workloads and don't provide any performance improvement for currently vectorized algorithms
@@ -90,7 +90,7 @@ inline constexpr size_t __native_vector_size = 1;
 #  endif
 
 template <class _ArithmeticT, size_t _Np>
-using __simd_vector __attribute__((__ext_vector_type__(_Np))) = _ArithmeticT;
+using __simd_vector __attribute__((__ext_vector_type__(_Np))) _LIBCPP_NODEBUG = _ArithmeticT;
 
 template <class _VecT>
 inline constexpr size_t __simd_vector_size_v = []<bool _False = false>() -> size_t {
@@ -106,7 +106,7 @@ _LIBCPP_HIDE_FROM_ABI _Tp __simd_vector_underlying_type_impl(__simd_vector<_Tp,
 }
 
 template <class _VecT>
-using __simd_vector_underlying_type_t = decltype(std::__simd_vector_underlying_type_impl(_VecT{}));
+using __simd_vector_underlying_type_t _LIBCPP_NODEBUG = decltype(std::__simd_vector_underlying_type_impl(_VecT{}));
 
 // This isn't inlined without always_inline when loading chars.
 template <class _VecT, class _Iter>
diff --git a/libcxx/include/__algorithm/sort.h b/libcxx/include/__algorithm/sort.h
index ed828b6d7231..5c60b23931cc 100644
--- a/libcxx/include/__algorithm/sort.h
+++ b/libcxx/include/__algorithm/sort.h
@@ -890,10 +890,10 @@ __sort_dispatch(_RandomAccessIterator __first, _RandomAccessIterator __last, _Co
 }
 
 template <class _Type, class... _Options>
-using __is_any_of = _Or<is_same<_Type, _Options>...>;
+using __is_any_of _LIBCPP_NODEBUG = _Or<is_same<_Type, _Options>...>;
 
 template <class _Type>
-using __sort_is_specialized_in_library = __is_any_of<
+using __sort_is_specialized_in_library _LIBCPP_NODEBUG = __is_any_of<
     _Type,
     char,
 #if _LIBCPP_HAS_WIDE_CHARACTERS
diff --git a/libcxx/include/__algorithm/three_way_comp_ref_type.h b/libcxx/include/__algorithm/three_way_comp_ref_type.h
index 5702a1fee082..f6f76455e466 100644
--- a/libcxx/include/__algorithm/three_way_comp_ref_type.h
+++ b/libcxx/include/__algorithm/three_way_comp_ref_type.h
@@ -61,10 +61,10 @@ struct __debug_three_way_comp {
 // Pass the comparator by lvalue reference. Or in the debug mode, using a debugging wrapper that stores a reference.
 #  if _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
 template <class _Comp>
-using __three_way_comp_ref_type = __debug_three_way_comp<_Comp>;
+using __three_way_comp_ref_type _LIBCPP_NODEBUG = __debug_three_way_comp<_Comp>;
 #  else
 template <class _Comp>
-using __three_way_comp_ref_type = _Comp&;
+using __three_way_comp_ref_type _LIBCPP_NODEBUG = _Comp&;
 #  endif
 
 #endif // _LIBCPP_STD_VER >= 20
diff --git a/libcxx/include/__algorithm/unwrap_iter.h b/libcxx/include/__algorithm/unwrap_iter.h
index 8cc0d22d4fc2..b66a682e765f 100644
--- a/libcxx/include/__algorithm/unwrap_iter.h
+++ b/libcxx/include/__algorithm/unwrap_iter.h
@@ -46,7 +46,7 @@ struct __unwrap_iter_impl {
 // It's a contiguous iterator, so we can use a raw pointer instead
 template <class _Iter>
 struct __unwrap_iter_impl<_Iter, true> {
-  using _ToAddressT = decltype(std::__to_address(std::declval<_Iter>()));
+  using _ToAddressT _LIBCPP_NODEBUG = decltype(std::__to_address(std::declval<_Iter>()));
 
   static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iter __rewrap(_Iter __orig_iter, _ToAddressT __unwrapped_iter) {
     return __orig_iter + (__unwrapped_iter - std::__to_address(__orig_iter));
diff --git a/libcxx/include/__atomic/aliases.h b/libcxx/include/__atomic/aliases.h
index 37d11dd0aabf..4fccebab2563 100644
--- a/libcxx/include/__atomic/aliases.h
+++ b/libcxx/include/__atomic/aliases.h
@@ -84,19 +84,19 @@ using atomic_uintmax_t = atomic<uintmax_t>;
 // C++20 atomic_{signed,unsigned}_lock_free: prefer the contention type most highly, then the largest lock-free type
 #if _LIBCPP_STD_VER >= 20
 #  if ATOMIC_LLONG_LOCK_FREE == 2
-using __largest_lock_free_type = long long;
+using __largest_lock_free_type _LIBCPP_NODEBUG = long long;
 #  elif ATOMIC_INT_LOCK_FREE == 2
-using __largest_lock_free_type = int;
+using __largest_lock_free_type _LIBCPP_NODEBUG = int;
 #  elif ATOMIC_SHORT_LOCK_FREE == 2
-using __largest_lock_free_type = short;
+using __largest_lock_free_type _LIBCPP_NODEBUG = short;
 #  elif ATOMIC_CHAR_LOCK_FREE == 2
-using __largest_lock_free_type = char;
+using __largest_lock_free_type _LIBCPP_NODEBUG = char;
 #  else
 #    define _LIBCPP_NO_LOCK_FREE_TYPES // There are no lockfree types (this can happen on unusual platforms)
 #  endif
 
 #  ifndef _LIBCPP_NO_LOCK_FREE_TYPES
-using __contention_t_or_largest =
+using __contention_t_or_largest _LIBCPP_NODEBUG =
     __conditional_t<__libcpp_is_always_lock_free<__cxx_contention_t>::__value,
                     __cxx_contention_t,
                     __largest_lock_free_type>;
diff --git a/libcxx/include/__atomic/atomic.h b/libcxx/include/__atomic/atomic.h
index 8029b52770d2..975a479e2040 100644
--- a/libcxx/include/__atomic/atomic.h
+++ b/libcxx/include/__atomic/atomic.h
@@ -143,7 +143,7 @@ struct __atomic_base // false
 
 template <class _Tp>
 struct __atomic_base<_Tp, true> : public __atomic_base<_Tp, false> {
-  using __base = __atomic_base<_Tp, false>;
+  using __base _LIBCPP_NODEBUG = __atomic_base<_Tp, false>;
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __atomic_base() _NOEXCEPT = default;
 
@@ -228,9 +228,9 @@ struct __atomic_waitable_traits<__atomic_base<_Tp, _IsIntegral> > {
 
 template <class _Tp>
 struct atomic : public __atomic_base<_Tp> {
-  using __base          = __atomic_base<_Tp>;
-  using value_type      = _Tp;
-  using difference_type = value_type;
+  using __base _LIBCPP_NODEBUG = __atomic_base<_Tp>;
+  using value_type             = _Tp;
+  using difference_type        = value_type;
 
 #if _LIBCPP_STD_VER >= 20
   _LIBCPP_HIDE_FROM_ABI atomic() = default;
@@ -257,9 +257,9 @@ struct atomic : public __atomic_base<_Tp> {
 
 template <class _Tp>
 struct atomic<_Tp*> : public __atomic_base<_Tp*> {
-  using __base          = __atomic_base<_Tp*>;
-  using value_type      = _Tp*;
-  using difference_type = ptrdiff_t;
+  using __base _LIBCPP_NODEBUG = __atomic_base<_Tp*>;
+  using value_type             = _Tp*;
+  using difference_type        = ptrdiff_t;
 
   _LIBCPP_HIDE_FROM_ABI atomic() _NOEXCEPT = default;
 
@@ -389,9 +389,9 @@ private:
   }
 
 public:
-  using __base          = __atomic_base<_Tp>;
-  using value_type      = _Tp;
-  using difference_type = value_type;
+  using __base _LIBCPP_NODEBUG = __atomic_base<_Tp>;
+  using value_type             = _Tp;
+  using difference_type        = value_type;
 
   _LIBCPP_HIDE_FROM_ABI constexpr atomic() noexcept = default;
   _LIBCPP_HIDE_FROM_ABI constexpr atomic(_Tp __d) noexcept : __base(__d) {}
diff --git a/libcxx/include/__atomic/atomic_ref.h b/libcxx/include/__atomic/atomic_ref.h
index eef15983b983..177ea646b6cd 100644
--- a/libcxx/include/__atomic/atomic_ref.h
+++ b/libcxx/include/__atomic/atomic_ref.h
@@ -221,7 +221,7 @@ public:
   _LIBCPP_HIDE_FROM_ABI void notify_all() const noexcept { std::__atomic_notify_all(*this); }
 
 protected:
-  using _Aligned_Tp [[__gnu__::__aligned__(required_alignment)]] = _Tp;
+  using _Aligned_Tp [[__gnu__::__aligned__(required_alignment), __gnu__::__nodebug__]] = _Tp;
   _Aligned_Tp* __ptr_;
 
   _LIBCPP_HIDE_FROM_ABI __atomic_ref_base(_Tp& __obj) : __ptr_(std::addressof(__obj)) {}
@@ -241,7 +241,7 @@ template <class _Tp>
 struct atomic_ref : public __atomic_ref_base<_Tp> {
   static_assert(is_trivially_copyable_v<_Tp>, "std::atomic_ref<T> requires that 'T' be a trivially copyable type");
 
-  using __base = __atomic_ref_base<_Tp>;
+  using __base _LIBCPP_NODEBUG = __atomic_ref_base<_Tp>;
 
   _LIBCPP_HIDE_FROM_ABI explicit atomic_ref(_Tp& __obj) : __base(__obj) {
     _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
@@ -259,7 +259,7 @@ struct atomic_ref : public __atomic_ref_base<_Tp> {
 template <class _Tp>
   requires(std::integral<_Tp> && !std::same_as<bool, _Tp>)
 struct atomic_ref<_Tp> : public __atomic_ref_base<_Tp> {
-  using __base = __atomic_ref_base<_Tp>;
+  using __base _LIBCPP_NODEBUG = __atomic_ref_base<_Tp>;
 
   using difference_type = __base::value_type;
 
@@ -305,7 +305,7 @@ struct atomic_ref<_Tp> : public __atomic_ref_base<_Tp> {
 template <class _Tp>
   requires std::floating_point<_Tp>
 struct atomic_ref<_Tp> : public __atomic_ref_base<_Tp> {
-  using __base = __atomic_ref_base<_Tp>;
+  using __base _LIBCPP_NODEBUG = __atomic_ref_base<_Tp>;
 
   using difference_type = __base::value_type;
 
@@ -344,7 +344,7 @@ struct atomic_ref<_Tp> : public __atomic_ref_base<_Tp> {
 
 template <class _Tp>
 struct atomic_ref<_Tp*> : public __atomic_ref_base<_Tp*> {
-  using __base = __atomic_ref_base<_Tp*>;
+  using __base _LIBCPP_NODEBUG = __atomic_ref_base<_Tp*>;
 
   using difference_type = ptrdiff_t;
 
diff --git a/libcxx/include/__atomic/atomic_sync.h b/libcxx/include/__atomic/atomic_sync.h
index 153001e7b62e..ab9bc59fdcfe 100644
--- a/libcxx/include/__atomic/atomic_sync.h
+++ b/libcxx/include/__atomic/atomic_sync.h
@@ -81,7 +81,7 @@ struct __atomic_wait_backoff_impl {
   _Poll __poll_;
   memory_order __order_;
 
-  using __waitable_traits = __atomic_waitable_traits<__decay_t<_AtomicWaitable> >;
+  using __waitable_traits _LIBCPP_NODEBUG = __atomic_waitable_traits<__decay_t<_AtomicWaitable> >;
 
   _LIBCPP_AVAILABILITY_SYNC
   _LIBCPP_HIDE_FROM_ABI bool
diff --git a/libcxx/include/__atomic/contention_t.h b/libcxx/include/__atomic/contention_t.h
index 6f2a073bc1a8..5b42a0125f87 100644
--- a/libcxx/include/__atomic/contention_t.h
+++ b/libcxx/include/__atomic/contention_t.h
@@ -20,12 +20,12 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if defined(__linux__) || (defined(_AIX) && !defined(__64BIT__))
-using __cxx_contention_t = int32_t;
+using __cxx_contention_t _LIBCPP_NODEBUG = int32_t;
 #else
-using __cxx_contention_t = int64_t;
+using __cxx_contention_t _LIBCPP_NODEBUG = int64_t;
 #endif // __linux__ || (_AIX && !__64BIT__)
 
-using __cxx_atomic_contention_t = __cxx_atomic_impl<__cxx_contention_t>;
+using __cxx_atomic_contention_t _LIBCPP_NODEBUG = __cxx_atomic_impl<__cxx_contention_t>;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__atomic/memory_order.h b/libcxx/include/__atomic/memory_order.h
index 294121d1c4e7..44790fe888b3 100644
--- a/libcxx/include/__atomic/memory_order.h
+++ b/libcxx/include/__atomic/memory_order.h
@@ -24,7 +24,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 // to pin the underlying type in C++20.
 enum __legacy_memory_order { __mo_relaxed, __mo_consume, __mo_acquire, __mo_release, __mo_acq_rel, __mo_seq_cst };
 
-using __memory_order_underlying_t = underlying_type<__legacy_memory_order>::type;
+using __memory_order_underlying_t _LIBCPP_NODEBUG = underlying_type<__legacy_memory_order>::type;
 
 #if _LIBCPP_STD_VER >= 20
 
diff --git a/libcxx/include/__bit_reference b/libcxx/include/__bit_reference
index 9fa24c98d493..7e27090cc68a 100644
--- a/libcxx/include/__bit_reference
+++ b/libcxx/include/__bit_reference
@@ -43,8 +43,8 @@ struct __has_storage_type {
 
 template <class _Cp, bool = __has_storage_type<_Cp>::value>
 class __bit_reference {
-  using __storage_type    = typename _Cp::__storage_type;
-  using __storage_pointer = typename _Cp::__storage_pointer;
+  using __storage_type _LIBCPP_NODEBUG    = typename _Cp::__storage_type;
+  using __storage_pointer _LIBCPP_NODEBUG = typename _Cp::__storage_pointer;
 
   __storage_pointer __seg_;
   __storage_type __mask_;
@@ -55,7 +55,7 @@ class __bit_reference {
   friend class __bit_iterator<_Cp, false>;
 
 public:
-  using __container = typename _Cp::__self;
+  using __container _LIBCPP_NODEBUG = typename _Cp::__self;
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bit_reference(const __bit_reference&) = default;
 
@@ -135,8 +135,8 @@ inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void swap(bool& __x,
 
 template <class _Cp>
 class __bit_const_reference {
-  using __storage_type    = typename _Cp::__storage_type;
-  using __storage_pointer = typename _Cp::__const_storage_pointer;
+  using __storage_type _LIBCPP_NODEBUG    = typename _Cp::__storage_type;
+  using __storage_pointer _LIBCPP_NODEBUG = typename _Cp::__const_storage_pointer;
 
   __storage_pointer __seg_;
   __storage_type __mask_;
@@ -145,7 +145,7 @@ class __bit_const_reference {
   friend class __bit_iterator<_Cp, true>;
 
 public:
-  using __container = typename _Cp::__self;
+  using __container _LIBCPP_NODEBUG = typename _Cp::__self;
 
   _LIBCPP_HIDE_FROM_ABI __bit_const_reference(const __bit_const_reference&) = default;
   __bit_const_reference& operator=(const __bit_const_reference&)            = delete;
@@ -587,10 +587,10 @@ inline _LIBCPP_HIDE_FROM_ABI __bit_iterator<_Cr, false> swap_ranges(
 
 template <class _Cp>
 struct __bit_array {
-  using difference_type   = typename _Cp::difference_type;
-  using __storage_type    = typename _Cp::__storage_type;
-  using __storage_pointer = typename _Cp::__storage_pointer;
-  using iterator          = typename _Cp::iterator;
+  using difference_type _LIBCPP_NODEBUG   = typename _Cp::difference_type;
+  using __storage_type _LIBCPP_NODEBUG    = typename _Cp::__storage_type;
+  using __storage_pointer _LIBCPP_NODEBUG = typename _Cp::__storage_pointer;
+  using iterator _LIBCPP_NODEBUG          = typename _Cp::iterator;
 
   static const unsigned __bits_per_word = _Cp::__bits_per_word;
   static const unsigned _Np             = 4;
@@ -790,8 +790,8 @@ public:
   using iterator_category = random_access_iterator_tag;
 
 private:
-  using __storage_type = typename _Cp::__storage_type;
-  using __storage_pointer =
+  using __storage_type _LIBCPP_NODEBUG = typename _Cp::__storage_type;
+  using __storage_pointer _LIBCPP_NODEBUG =
       __conditional_t<_IsConst, typename _Cp::__const_storage_pointer, typename _Cp::__storage_pointer>;
 
   static const unsigned __bits_per_word = _Cp::__bits_per_word;
diff --git a/libcxx/include/__chrono/formatter.h b/libcxx/include/__chrono/formatter.h
index 3671e6aa52b4..1086dde38195 100644
--- a/libcxx/include/__chrono/formatter.h
+++ b/libcxx/include/__chrono/formatter.h
@@ -711,7 +711,7 @@ public:
 template <class _Duration, __fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::sys_time<_Duration>, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -722,7 +722,7 @@ public:
 template <class _Duration, __fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::file_time<_Duration>, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -733,7 +733,7 @@ public:
 template <class _Duration, __fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::local_time<_Duration>, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -745,7 +745,7 @@ public:
 template <class _Rep, class _Period, __fmt_char_type _CharT>
 struct formatter<chrono::duration<_Rep, _Period>, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -767,7 +767,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::day, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -778,7 +778,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::month, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -789,7 +789,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::year, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -800,7 +800,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::weekday, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -811,7 +811,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::weekday_indexed, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -822,7 +822,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::weekday_last, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -833,7 +833,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::month_day, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -844,7 +844,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::month_day_last, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -855,7 +855,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::month_weekday, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -866,7 +866,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::month_weekday_last, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -877,7 +877,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::year_month, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -888,7 +888,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::year_month_day, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -899,7 +899,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::year_month_day_last, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -910,7 +910,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::year_month_weekday, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -921,7 +921,7 @@ public:
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<chrono::year_month_weekday_last, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -932,7 +932,7 @@ public:
 template <class _Duration, __fmt_char_type _CharT>
 struct formatter<chrono::hh_mm_ss<_Duration>, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -944,7 +944,7 @@ public:
 template <__fmt_char_type _CharT>
 struct formatter<chrono::sys_info, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -955,7 +955,7 @@ public:
 template <__fmt_char_type _CharT>
 struct formatter<chrono::local_info, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
@@ -968,7 +968,7 @@ public:
 template <class _Duration, class _TimeZonePtr, __fmt_char_type _CharT>
 struct formatter<chrono::zoned_time<_Duration, _TimeZonePtr>, _CharT> : public __formatter_chrono<_CharT> {
 public:
-  using _Base = __formatter_chrono<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
diff --git a/libcxx/include/__chrono/hh_mm_ss.h b/libcxx/include/__chrono/hh_mm_ss.h
index c460b1130508..6ea8a28ee093 100644
--- a/libcxx/include/__chrono/hh_mm_ss.h
+++ b/libcxx/include/__chrono/hh_mm_ss.h
@@ -30,7 +30,7 @@ template <class _Duration>
 class hh_mm_ss {
 private:
   static_assert(__is_duration_v<_Duration>, "template parameter of hh_mm_ss must be a std::chrono::duration");
-  using __CommonType = common_type_t<_Duration, chrono::seconds>;
+  using __CommonType _LIBCPP_NODEBUG = common_type_t<_Duration, chrono::seconds>;
 
   _LIBCPP_HIDE_FROM_ABI static constexpr uint64_t __pow10(unsigned __exp) {
     uint64_t __ret = 1;
diff --git a/libcxx/include/__chrono/parser_std_format_spec.h b/libcxx/include/__chrono/parser_std_format_spec.h
index 3976864c12b9..4df8e603c6bc 100644
--- a/libcxx/include/__chrono/parser_std_format_spec.h
+++ b/libcxx/include/__chrono/parser_std_format_spec.h
@@ -140,7 +140,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr void __validate_time_zone(__flags __flags) {
 
 template <class _CharT>
 class _LIBCPP_TEMPLATE_VIS __parser_chrono {
-  using _ConstIterator = typename basic_format_parse_context<_CharT>::const_iterator;
+  using _ConstIterator _LIBCPP_NODEBUG = typename basic_format_parse_context<_CharT>::const_iterator;
 
 public:
   template <class _ParseContext>
diff --git a/libcxx/include/__chrono/zoned_time.h b/libcxx/include/__chrono/zoned_time.h
index f57e65c90a62..1deba10d96ae 100644
--- a/libcxx/include/__chrono/zoned_time.h
+++ b/libcxx/include/__chrono/zoned_time.h
@@ -66,7 +66,7 @@ class zoned_time {
   // Using these constraints in the code causes the compiler to give an
   // error that the constraint depends on itself. To avoid that issue use
   // the fact it is possible to create this object from a _TimeZonePtr.
-  using __traits = zoned_traits<_TimeZonePtr>;
+  using __traits _LIBCPP_NODEBUG = zoned_traits<_TimeZonePtr>;
 
 public:
   using duration = common_type_t<_Duration, seconds>;
@@ -186,7 +186,7 @@ template <class _Duration>
 zoned_time(sys_time<_Duration>) -> zoned_time<common_type_t<_Duration, seconds>>;
 
 template <class _TimeZonePtrOrName>
-using __time_zone_representation =
+using __time_zone_representation _LIBCPP_NODEBUG =
     conditional_t<is_convertible_v<_TimeZonePtrOrName, string_view>,
                   const time_zone*,
                   remove_cvref_t<_TimeZonePtrOrName>>;
diff --git a/libcxx/include/__compare/ordering.h b/libcxx/include/__compare/ordering.h
index 297218e6f29e..902ef5329dd4 100644
--- a/libcxx/include/__compare/ordering.h
+++ b/libcxx/include/__compare/ordering.h
@@ -120,7 +120,7 @@ inline constexpr partial_ordering partial_ordering::greater(_PartialOrdResult::_
 inline constexpr partial_ordering partial_ordering::unordered(_PartialOrdResult::__unordered);
 
 class weak_ordering {
-  using _ValueT = signed char;
+  using _ValueT _LIBCPP_NODEBUG = signed char;
 
   _LIBCPP_HIDE_FROM_ABI explicit constexpr weak_ordering(_OrdResult __v) noexcept : __value_(_ValueT(__v)) {}
 
@@ -190,7 +190,7 @@ inline constexpr weak_ordering weak_ordering::equivalent(_OrdResult::__equiv);
 inline constexpr weak_ordering weak_ordering::greater(_OrdResult::__greater);
 
 class strong_ordering {
-  using _ValueT = signed char;
+  using _ValueT _LIBCPP_NODEBUG = signed char;
 
   _LIBCPP_HIDE_FROM_ABI explicit constexpr strong_ordering(_OrdResult __v) noexcept : __value_(_ValueT(__v)) {}
 
diff --git a/libcxx/include/__compare/synth_three_way.h b/libcxx/include/__compare/synth_three_way.h
index e48ce4979983..63bf56d0cf42 100644
--- a/libcxx/include/__compare/synth_three_way.h
+++ b/libcxx/include/__compare/synth_three_way.h
@@ -43,7 +43,8 @@ _LIBCPP_HIDE_FROM_ABI inline constexpr auto __synth_three_way = []<class _Tp, cl
 };
 
 template <class _Tp, class _Up = _Tp>
-using __synth_three_way_result = decltype(std::__synth_three_way(std::declval<_Tp&>(), std::declval<_Up&>()));
+using __synth_three_way_result _LIBCPP_NODEBUG =
+    decltype(std::__synth_three_way(std::declval<_Tp&>(), std::declval<_Up&>()));
 
 #endif // _LIBCPP_STD_VER >= 20
 
diff --git a/libcxx/include/__exception/exception_ptr.h b/libcxx/include/__exception/exception_ptr.h
index 7df46a0e94dc..6257e6f729bf 100644
--- a/libcxx/include/__exception/exception_ptr.h
+++ b/libcxx/include/__exception/exception_ptr.h
@@ -66,7 +66,7 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
 
 public:
   // exception_ptr is basically a COW string.
-  using __trivially_relocatable = exception_ptr;
+  using __trivially_relocatable _LIBCPP_NODEBUG = exception_ptr;
 
   _LIBCPP_HIDE_FROM_ABI exception_ptr() _NOEXCEPT : __ptr_() {}
   _LIBCPP_HIDE_FROM_ABI exception_ptr(nullptr_t) _NOEXCEPT : __ptr_() {}
diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index 3d3f11967ee7..03bbd1623ed5 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -459,14 +459,14 @@ class expected : private __expected_base<_Tp, _Err> {
   template <class _Up, class _OtherErr>
   friend class expected;
 
-  using __base = __expected_base<_Tp, _Err>;
+  using __base _LIBCPP_NODEBUG = __expected_base<_Tp, _Err>;
 
 public:
   using value_type      = _Tp;
   using error_type      = _Err;
   using unexpected_type = unexpected<_Err>;
 
-  using __trivially_relocatable =
+  using __trivially_relocatable _LIBCPP_NODEBUG =
       __conditional_t<__libcpp_is_trivially_relocatable<_Tp>::value && __libcpp_is_trivially_relocatable<_Err>::value,
                       expected,
                       void>;
@@ -505,7 +505,7 @@ public:
 
 private:
   template <class _Up, class _OtherErr, class _UfQual, class _OtherErrQual>
-  using __can_convert = _And<
+  using __can_convert _LIBCPP_NODEBUG = _And<
       is_constructible<_Tp, _UfQual>,
       is_constructible<_Err, _OtherErrQual>,
       _If<_Not<is_same<remove_cv_t<_Tp>, bool>>::value,
@@ -1363,7 +1363,7 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
   friend class expected;
 
   template <class _Up, class _OtherErr, class _OtherErrQual>
-  using __can_convert =
+  using __can_convert _LIBCPP_NODEBUG =
       _And< is_void<_Up>,
             is_constructible<_Err, _OtherErrQual>,
             _Not<is_constructible<unexpected<_Err>, expected<_Up, _OtherErr>&>>,
@@ -1371,7 +1371,7 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
             _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>&>>,
             _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>>>>;
 
-  using __base = __expected_void_base<_Err>;
+  using __base _LIBCPP_NODEBUG = __expected_void_base<_Err>;
 
 public:
   using value_type      = _Tp;
diff --git a/libcxx/include/__expected/unexpected.h b/libcxx/include/__expected/unexpected.h
index cf110bcf69a8..6904889b8c6b 100644
--- a/libcxx/include/__expected/unexpected.h
+++ b/libcxx/include/__expected/unexpected.h
@@ -48,12 +48,12 @@ template <class _Err>
 struct __is_std_unexpected<unexpected<_Err>> : true_type {};
 
 template <class _Tp>
-using __valid_std_unexpected = _BoolConstant< //
-    is_object_v<_Tp> &&                       //
-    !is_array_v<_Tp> &&                       //
-    !__is_std_unexpected<_Tp>::value &&       //
-    !is_const_v<_Tp> &&                       //
-    !is_volatile_v<_Tp>                       //
+using __valid_std_unexpected _LIBCPP_NODEBUG = _BoolConstant< //
+    is_object_v<_Tp> &&                                       //
+    !is_array_v<_Tp> &&                                       //
+    !__is_std_unexpected<_Tp>::value &&                       //
+    !is_const_v<_Tp> &&                                       //
+    !is_volatile_v<_Tp>                                       //
     >;
 
 template <class _Err>
diff --git a/libcxx/include/__filesystem/directory_entry.h b/libcxx/include/__filesystem/directory_entry.h
index 7d0c01b98def..11e07acdbe00 100644
--- a/libcxx/include/__filesystem/directory_entry.h
+++ b/libcxx/include/__filesystem/directory_entry.h
@@ -22,7 +22,9 @@
 #include <__filesystem/perms.h>
 #include <__fwd/ostream.h>
 #include <__system_error/errc.h>
+#include <__system_error/error_category.h>
 #include <__system_error/error_code.h>
+#include <__system_error/error_condition.h>
 #include <__utility/move.h>
 #include <__utility/unreachable.h>
 #include <cstdint>
@@ -274,15 +276,7 @@ private:
   _LIBCPP_EXPORTED_FROM_ABI error_code __do_refresh() noexcept;
 
   _LIBCPP_HIDE_FROM_ABI static bool __is_dne_error(error_code const& __ec) {
-    if (!__ec)
-      return true;
-    switch (static_cast<errc>(__ec.value())) {
-    case errc::no_such_file_or_directory:
-    case errc::not_a_directory:
-      return true;
-    default:
-      return false;
-    }
+    return !__ec || __ec == errc::no_such_file_or_directory || __ec == errc::not_a_directory;
   }
 
   _LIBCPP_HIDE_FROM_ABI void
diff --git a/libcxx/include/__filesystem/path.h b/libcxx/include/__filesystem/path.h
index 509d1cc8052f..0a751ba32954 100644
--- a/libcxx/include/__filesystem/path.h
+++ b/libcxx/include/__filesystem/path.h
@@ -51,30 +51,30 @@ template <class _Tp>
 struct __can_convert_char<const _Tp> : public __can_convert_char<_Tp> {};
 template <>
 struct __can_convert_char<char> {
-  static const bool value = true;
-  using __char_type       = char;
+  static const bool value           = true;
+  using __char_type _LIBCPP_NODEBUG = char;
 };
 template <>
 struct __can_convert_char<wchar_t> {
-  static const bool value = true;
-  using __char_type       = wchar_t;
+  static const bool value           = true;
+  using __char_type _LIBCPP_NODEBUG = wchar_t;
 };
 #  if _LIBCPP_HAS_CHAR8_T
 template <>
 struct __can_convert_char<char8_t> {
-  static const bool value = true;
-  using __char_type       = char8_t;
+  static const bool value           = true;
+  using __char_type _LIBCPP_NODEBUG = char8_t;
 };
 #  endif
 template <>
 struct __can_convert_char<char16_t> {
-  static const bool value = true;
-  using __char_type       = char16_t;
+  static const bool value           = true;
+  using __char_type _LIBCPP_NODEBUG = char16_t;
 };
 template <>
 struct __can_convert_char<char32_t> {
-  static const bool value = true;
-  using __char_type       = char32_t;
+  static const bool value           = true;
+  using __char_type _LIBCPP_NODEBUG = char32_t;
 };
 
 template <class _ECharT, __enable_if_t<__can_convert_char<_ECharT>::value, int> = 0>
@@ -95,7 +95,7 @@ typedef string __u8_string;
 struct _NullSentinel {};
 
 template <class _Tp>
-using _Void = void;
+using _Void _LIBCPP_NODEBUG = void;
 
 template <class _Tp, class = void>
 struct __is_pathable_string : public false_type {};
@@ -104,7 +104,7 @@ template <class _ECharT, class _Traits, class _Alloc>
 struct __is_pathable_string< basic_string<_ECharT, _Traits, _Alloc>,
                              _Void<typename __can_convert_char<_ECharT>::__char_type> >
     : public __can_convert_char<_ECharT> {
-  using _Str = basic_string<_ECharT, _Traits, _Alloc>;
+  using _Str _LIBCPP_NODEBUG = basic_string<_ECharT, _Traits, _Alloc>;
 
   _LIBCPP_HIDE_FROM_ABI static _ECharT const* __range_begin(_Str const& __s) { return __s.data(); }
 
@@ -117,7 +117,7 @@ template <class _ECharT, class _Traits>
 struct __is_pathable_string< basic_string_view<_ECharT, _Traits>,
                              _Void<typename __can_convert_char<_ECharT>::__char_type> >
     : public __can_convert_char<_ECharT> {
-  using _Str = basic_string_view<_ECharT, _Traits>;
+  using _Str _LIBCPP_NODEBUG = basic_string_view<_ECharT, _Traits>;
 
   _LIBCPP_HIDE_FROM_ABI static _ECharT const* __range_begin(_Str const& __s) { return __s.data(); }
 
@@ -157,7 +157,7 @@ struct __is_pathable_iter<
     true,
     _Void<typename __can_convert_char< typename iterator_traits<_Iter>::value_type>::__char_type> >
     : __can_convert_char<typename iterator_traits<_Iter>::value_type> {
-  using _ECharT = typename iterator_traits<_Iter>::value_type;
+  using _ECharT _LIBCPP_NODEBUG = typename iterator_traits<_Iter>::value_type;
 
   _LIBCPP_HIDE_FROM_ABI static _Iter __range_begin(_Iter __b) { return __b; }
 
@@ -380,13 +380,13 @@ struct _PathExport<char8_t> {
 
 class _LIBCPP_EXPORTED_FROM_ABI path {
   template <class _SourceOrIter, class _Tp = path&>
-  using _EnableIfPathable = __enable_if_t<__is_pathable<_SourceOrIter>::value, _Tp>;
+  using _EnableIfPathable _LIBCPP_NODEBUG = __enable_if_t<__is_pathable<_SourceOrIter>::value, _Tp>;
 
   template <class _Tp>
-  using _SourceChar = typename __is_pathable<_Tp>::__char_type;
+  using _SourceChar _LIBCPP_NODEBUG = typename __is_pathable<_Tp>::__char_type;
 
   template <class _Tp>
-  using _SourceCVT = _PathCVT<_SourceChar<_Tp> >;
+  using _SourceCVT _LIBCPP_NODEBUG = _PathCVT<_SourceChar<_Tp> >;
 
 public:
 #  if defined(_LIBCPP_WIN32API)
diff --git a/libcxx/include/__flat_map/flat_map.h b/libcxx/include/__flat_map/flat_map.h
index b66bc1cb66fc..9fe84250b120 100644
--- a/libcxx/include/__flat_map/flat_map.h
+++ b/libcxx/include/__flat_map/flat_map.h
@@ -90,7 +90,7 @@ class flat_map {
   static_assert(!is_same_v<_MappedContainer, std::vector<bool>>, "vector<bool> is not a sequence container");
 
   template <bool _Const>
-  using __iterator = __key_value_iterator<flat_map, _KeyContainer, _MappedContainer, _Const>;
+  using __iterator _LIBCPP_NODEBUG = __key_value_iterator<flat_map, _KeyContainer, _MappedContainer, _Const>;
 
 public:
   // types
diff --git a/libcxx/include/__flat_map/key_value_iterator.h b/libcxx/include/__flat_map/key_value_iterator.h
index 987ac677a413..06a23f342997 100644
--- a/libcxx/include/__flat_map/key_value_iterator.h
+++ b/libcxx/include/__flat_map/key_value_iterator.h
@@ -41,9 +41,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Owner, class _KeyContainer, class _MappedContainer, bool _Const>
 struct __key_value_iterator {
 private:
-  using __key_iterator    = ranges::iterator_t<const _KeyContainer>;
-  using __mapped_iterator = ranges::iterator_t<__maybe_const<_Const, _MappedContainer>>;
-  using __reference       = _If<_Const, typename _Owner::const_reference, typename _Owner::reference>;
+  using __key_iterator _LIBCPP_NODEBUG    = ranges::iterator_t<const _KeyContainer>;
+  using __mapped_iterator _LIBCPP_NODEBUG = ranges::iterator_t<__maybe_const<_Const, _MappedContainer>>;
+  using __reference _LIBCPP_NODEBUG       = _If<_Const, typename _Owner::const_reference, typename _Owner::reference>;
 
   struct __arrow_proxy {
     __reference __ref_;
diff --git a/libcxx/include/__format/buffer.h b/libcxx/include/__format/buffer.h
index 618b8ef02564..9509f19e1672 100644
--- a/libcxx/include/__format/buffer.h
+++ b/libcxx/include/__format/buffer.h
@@ -322,7 +322,7 @@ struct _LIBCPP_TEMPLATE_VIS __back_insert_iterator_container<back_insert_iterato
 template <class _Container>
 class _LIBCPP_TEMPLATE_VIS __writer_container {
 public:
-  using _CharT = typename _Container::value_type;
+  using _CharT _LIBCPP_NODEBUG = typename _Container::value_type;
 
   _LIBCPP_HIDE_FROM_ABI explicit __writer_container(back_insert_iterator<_Container> __out_it)
       : __container_{__out_it.__get_container()} {}
@@ -340,7 +340,7 @@ private:
 /// Selects the type of the writer used for the output iterator.
 template <class _OutIt, class _CharT>
 class _LIBCPP_TEMPLATE_VIS __writer_selector {
-  using _Container = typename __back_insert_iterator_container<_OutIt>::type;
+  using _Container _LIBCPP_NODEBUG = typename __back_insert_iterator_container<_OutIt>::type;
 
 public:
   using type =
@@ -355,7 +355,7 @@ public:
 template <class _OutIt, __fmt_char_type _CharT>
   requires(output_iterator<_OutIt, const _CharT&>)
 class _LIBCPP_TEMPLATE_VIS __format_buffer {
-  using _Storage =
+  using _Storage _LIBCPP_NODEBUG =
       conditional_t<__enable_direct_output<_OutIt, _CharT>, __direct_storage<_CharT>, __internal_storage<_CharT>>;
 
 public:
@@ -408,7 +408,7 @@ private:
 template <class _OutIt, __fmt_char_type _CharT, bool>
   requires(output_iterator<_OutIt, const _CharT&>)
 struct _LIBCPP_TEMPLATE_VIS __format_to_n_buffer_base {
-  using _Size = iter_difference_t<_OutIt>;
+  using _Size _LIBCPP_NODEBUG = iter_difference_t<_OutIt>;
 
 public:
   _LIBCPP_HIDE_FROM_ABI explicit __format_to_n_buffer_base(_OutIt __out_it, _Size __max_size)
@@ -438,7 +438,7 @@ protected:
 template <class _OutIt, __fmt_char_type _CharT>
   requires(output_iterator<_OutIt, const _CharT&>)
 class _LIBCPP_TEMPLATE_VIS __format_to_n_buffer_base<_OutIt, _CharT, true> {
-  using _Size = iter_difference_t<_OutIt>;
+  using _Size _LIBCPP_NODEBUG = iter_difference_t<_OutIt>;
 
 public:
   _LIBCPP_HIDE_FROM_ABI explicit __format_to_n_buffer_base(_OutIt __out_it, _Size __max_size)
@@ -489,8 +489,8 @@ template <class _OutIt, __fmt_char_type _CharT>
   requires(output_iterator<_OutIt, const _CharT&>)
 struct _LIBCPP_TEMPLATE_VIS __format_to_n_buffer final
     : public __format_to_n_buffer_base< _OutIt, _CharT, __enable_direct_output<_OutIt, _CharT>> {
-  using _Base = __format_to_n_buffer_base<_OutIt, _CharT, __enable_direct_output<_OutIt, _CharT>>;
-  using _Size = iter_difference_t<_OutIt>;
+  using _Base _LIBCPP_NODEBUG = __format_to_n_buffer_base<_OutIt, _CharT, __enable_direct_output<_OutIt, _CharT>>;
+  using _Size _LIBCPP_NODEBUG = iter_difference_t<_OutIt>;
 
 public:
   _LIBCPP_HIDE_FROM_ABI explicit __format_to_n_buffer(_OutIt __out_it, _Size __max_size)
@@ -523,7 +523,7 @@ public:
 // would lead to a circular include with formatter for vector<bool>.
 template <__fmt_char_type _CharT>
 class _LIBCPP_TEMPLATE_VIS __retarget_buffer {
-  using _Alloc = allocator<_CharT>;
+  using _Alloc _LIBCPP_NODEBUG = allocator<_CharT>;
 
 public:
   using value_type = _CharT;
diff --git a/libcxx/include/__format/concepts.h b/libcxx/include/__format/concepts.h
index 2c40e3e31491..28297c612db7 100644
--- a/libcxx/include/__format/concepts.h
+++ b/libcxx/include/__format/concepts.h
@@ -44,7 +44,7 @@ concept __fmt_char_type =
 // (Note testing for (w)format_context would be a valid choice, but requires
 // selecting the proper one depending on the type of _CharT.)
 template <class _CharT>
-using __fmt_iter_for = _CharT*;
+using __fmt_iter_for _LIBCPP_NODEBUG = _CharT*;
 
 template <class _Tp, class _Context, class _Formatter = typename _Context::template formatter_type<remove_const_t<_Tp>>>
 concept __formattable_with =
diff --git a/libcxx/include/__format/container_adaptor.h b/libcxx/include/__format/container_adaptor.h
index d3be2e189560..48d42ee7d901 100644
--- a/libcxx/include/__format/container_adaptor.h
+++ b/libcxx/include/__format/container_adaptor.h
@@ -37,8 +37,8 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Adaptor, class _CharT>
 struct _LIBCPP_TEMPLATE_VIS __formatter_container_adaptor {
 private:
-  using __maybe_const_container = __fmt_maybe_const<typename _Adaptor::container_type, _CharT>;
-  using __maybe_const_adaptor   = __maybe_const<is_const_v<__maybe_const_container>, _Adaptor>;
+  using __maybe_const_container _LIBCPP_NODEBUG = __fmt_maybe_const<typename _Adaptor::container_type, _CharT>;
+  using __maybe_const_adaptor _LIBCPP_NODEBUG   = __maybe_const<is_const_v<__maybe_const_container>, _Adaptor>;
   formatter<ranges::ref_view<__maybe_const_container>, _CharT> __underlying_;
 
 public:
diff --git a/libcxx/include/__format/format_arg.h b/libcxx/include/__format/format_arg.h
index a973ccd43c42..1c530fd5a5d0 100644
--- a/libcxx/include/__format/format_arg.h
+++ b/libcxx/include/__format/format_arg.h
@@ -208,7 +208,7 @@ _LIBCPP_HIDE_FROM_ABI _Rp __visit_format_arg(_Visitor&& __vis, basic_format_arg<
 /// separate arrays.
 template <class _Context>
 class __basic_format_arg_value {
-  using _CharT = typename _Context::char_type;
+  using _CharT _LIBCPP_NODEBUG = typename _Context::char_type;
 
 public:
   /// Contains the implementation for basic_format_arg::handle.
diff --git a/libcxx/include/__format/format_arg_store.h b/libcxx/include/__format/format_arg_store.h
index 8b2c95c657c9..4c5ee9e9e4fd 100644
--- a/libcxx/include/__format/format_arg_store.h
+++ b/libcxx/include/__format/format_arg_store.h
@@ -257,7 +257,7 @@ struct _LIBCPP_TEMPLATE_VIS __format_arg_store {
     }
   }
 
-  using _Storage =
+  using _Storage _LIBCPP_NODEBUG =
       conditional_t<__format::__use_packed_format_arg_store(sizeof...(_Args)),
                     __format::__packed_format_arg_store<_Context, sizeof...(_Args)>,
                     __format::__unpacked_format_arg_store<_Context, sizeof...(_Args)>>;
diff --git a/libcxx/include/__format/format_functions.h b/libcxx/include/__format/format_functions.h
index 3991363c0124..b920be5acbe8 100644
--- a/libcxx/include/__format/format_functions.h
+++ b/libcxx/include/__format/format_functions.h
@@ -379,7 +379,7 @@ struct _LIBCPP_TEMPLATE_VIS basic_format_string {
 private:
   basic_string_view<_CharT> __str_;
 
-  using _Context = __format::__compile_time_basic_format_context<_CharT>;
+  using _Context _LIBCPP_NODEBUG = __format::__compile_time_basic_format_context<_CharT>;
 
   static constexpr array<__format::__arg_t, sizeof...(_Args)> __types_{
       __format::__determine_arg_t<_Context, remove_cvref_t<_Args>>()...};
diff --git a/libcxx/include/__format/formatter_floating_point.h b/libcxx/include/__format/formatter_floating_point.h
index e04fffb683c3..ac4be9b61935 100644
--- a/libcxx/include/__format/formatter_floating_point.h
+++ b/libcxx/include/__format/formatter_floating_point.h
@@ -141,7 +141,7 @@ struct __traits<double> {
 /// on the stack or the heap.
 template <floating_point _Fp>
 class _LIBCPP_TEMPLATE_VIS __float_buffer {
-  using _Traits = __traits<_Fp>;
+  using _Traits _LIBCPP_NODEBUG = __traits<_Fp>;
 
 public:
   // TODO FMT Improve this constructor to do a better estimate.
diff --git a/libcxx/include/__format/formatter_string.h b/libcxx/include/__format/formatter_string.h
index 826d6421c863..30084e582214 100644
--- a/libcxx/include/__format/formatter_string.h
+++ b/libcxx/include/__format/formatter_string.h
@@ -59,7 +59,7 @@ public:
 // Formatter const char*.
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<const _CharT*, _CharT> : public __formatter_string<_CharT> {
-  using _Base = __formatter_string<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_string<_CharT>;
 
   template <class _FormatContext>
   _LIBCPP_HIDE_FROM_ABI typename _FormatContext::iterator format(const _CharT* __str, _FormatContext& __ctx) const {
@@ -78,7 +78,7 @@ struct _LIBCPP_TEMPLATE_VIS formatter<const _CharT*, _CharT> : public __formatte
 // Formatter char*.
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<_CharT*, _CharT> : public formatter<const _CharT*, _CharT> {
-  using _Base = formatter<const _CharT*, _CharT>;
+  using _Base _LIBCPP_NODEBUG = formatter<const _CharT*, _CharT>;
 
   template <class _FormatContext>
   _LIBCPP_HIDE_FROM_ABI typename _FormatContext::iterator format(_CharT* __str, _FormatContext& __ctx) const {
@@ -89,7 +89,7 @@ struct _LIBCPP_TEMPLATE_VIS formatter<_CharT*, _CharT> : public formatter<const
 // Formatter char[].
 template <__fmt_char_type _CharT, size_t _Size>
 struct _LIBCPP_TEMPLATE_VIS formatter<_CharT[_Size], _CharT> : public __formatter_string<_CharT> {
-  using _Base = __formatter_string<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_string<_CharT>;
 
   template <class _FormatContext>
   _LIBCPP_HIDE_FROM_ABI typename _FormatContext::iterator
@@ -102,7 +102,7 @@ struct _LIBCPP_TEMPLATE_VIS formatter<_CharT[_Size], _CharT> : public __formatte
 template <__fmt_char_type _CharT, class _Traits, class _Allocator>
 struct _LIBCPP_TEMPLATE_VIS formatter<basic_string<_CharT, _Traits, _Allocator>, _CharT>
     : public __formatter_string<_CharT> {
-  using _Base = __formatter_string<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_string<_CharT>;
 
   template <class _FormatContext>
   _LIBCPP_HIDE_FROM_ABI typename _FormatContext::iterator
@@ -115,7 +115,7 @@ struct _LIBCPP_TEMPLATE_VIS formatter<basic_string<_CharT, _Traits, _Allocator>,
 // Formatter std::string_view.
 template <__fmt_char_type _CharT, class _Traits>
 struct _LIBCPP_TEMPLATE_VIS formatter<basic_string_view<_CharT, _Traits>, _CharT> : public __formatter_string<_CharT> {
-  using _Base = __formatter_string<_CharT>;
+  using _Base _LIBCPP_NODEBUG = __formatter_string<_CharT>;
 
   template <class _FormatContext>
   _LIBCPP_HIDE_FROM_ABI typename _FormatContext::iterator
diff --git a/libcxx/include/__format/range_default_formatter.h b/libcxx/include/__format/range_default_formatter.h
index fb21b0f8beb3..bb4c520f5ea1 100644
--- a/libcxx/include/__format/range_default_formatter.h
+++ b/libcxx/include/__format/range_default_formatter.h
@@ -40,7 +40,7 @@ concept __const_formattable_range =
     ranges::input_range<const _Rp> && formattable<ranges::range_reference_t<const _Rp>, _CharT>;
 
 template <class _Rp, class _CharT>
-using __fmt_maybe_const = conditional_t<__const_formattable_range<_Rp, _CharT>, const _Rp, _Rp>;
+using __fmt_maybe_const _LIBCPP_NODEBUG = conditional_t<__const_formattable_range<_Rp, _CharT>, const _Rp, _Rp>;
 
 _LIBCPP_DIAGNOSTIC_PUSH
 _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wshadow")
@@ -95,7 +95,7 @@ struct _LIBCPP_TEMPLATE_VIS __range_default_formatter;
 template <ranges::input_range _Rp, class _CharT>
 struct _LIBCPP_TEMPLATE_VIS __range_default_formatter<range_format::sequence, _Rp, _CharT> {
 private:
-  using __maybe_const_r = __fmt_maybe_const<_Rp, _CharT>;
+  using __maybe_const_r _LIBCPP_NODEBUG = __fmt_maybe_const<_Rp, _CharT>;
   range_formatter<remove_cvref_t<ranges::range_reference_t<__maybe_const_r>>, _CharT> __underlying_;
 
 public:
@@ -122,8 +122,8 @@ public:
 template <ranges::input_range _Rp, class _CharT>
 struct _LIBCPP_TEMPLATE_VIS __range_default_formatter<range_format::map, _Rp, _CharT> {
 private:
-  using __maybe_const_map = __fmt_maybe_const<_Rp, _CharT>;
-  using __element_type    = remove_cvref_t<ranges::range_reference_t<__maybe_const_map>>;
+  using __maybe_const_map _LIBCPP_NODEBUG = __fmt_maybe_const<_Rp, _CharT>;
+  using __element_type _LIBCPP_NODEBUG    = remove_cvref_t<ranges::range_reference_t<__maybe_const_map>>;
   range_formatter<__element_type, _CharT> __underlying_;
 
 public:
@@ -150,8 +150,8 @@ public:
 template <ranges::input_range _Rp, class _CharT>
 struct _LIBCPP_TEMPLATE_VIS __range_default_formatter<range_format::set, _Rp, _CharT> {
 private:
-  using __maybe_const_set = __fmt_maybe_const<_Rp, _CharT>;
-  using __element_type    = remove_cvref_t<ranges::range_reference_t<__maybe_const_set>>;
+  using __maybe_const_set _LIBCPP_NODEBUG = __fmt_maybe_const<_Rp, _CharT>;
+  using __element_type _LIBCPP_NODEBUG    = remove_cvref_t<ranges::range_reference_t<__maybe_const_set>>;
   range_formatter<__element_type, _CharT> __underlying_;
 
 public:
diff --git a/libcxx/include/__format/unicode.h b/libcxx/include/__format/unicode.h
index b4f22c739d6a..46096fda1e8a 100644
--- a/libcxx/include/__format/unicode.h
+++ b/libcxx/include/__format/unicode.h
@@ -123,7 +123,7 @@ class __code_point_view;
 /// UTF-8 specialization.
 template <>
 class __code_point_view<char> {
-  using _Iterator = basic_string_view<char>::const_iterator;
+  using _Iterator _LIBCPP_NODEBUG = basic_string_view<char>::const_iterator;
 
 public:
   _LIBCPP_HIDE_FROM_ABI constexpr explicit __code_point_view(_Iterator __first, _Iterator __last)
@@ -249,7 +249,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr bool __is_surrogate_pair_low(wchar_t __value) {
 /// - 4 UTF-32 (for example Linux)
 template <>
 class __code_point_view<wchar_t> {
-  using _Iterator = typename basic_string_view<wchar_t>::const_iterator;
+  using _Iterator _LIBCPP_NODEBUG = typename basic_string_view<wchar_t>::const_iterator;
 
 public:
   static_assert(sizeof(wchar_t) == 2 || sizeof(wchar_t) == 4, "sizeof(wchar_t) has a not implemented value");
@@ -300,8 +300,8 @@ private:
 // This implements the extended rules see
 // https://www.unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries
 class __extended_grapheme_cluster_break {
-  using __EGC_property  = __extended_grapheme_custer_property_boundary::__property;
-  using __inCB_property = __indic_conjunct_break::__property;
+  using __EGC_property _LIBCPP_NODEBUG  = __extended_grapheme_custer_property_boundary::__property;
+  using __inCB_property _LIBCPP_NODEBUG = __indic_conjunct_break::__property;
 
 public:
   _LIBCPP_HIDE_FROM_ABI constexpr explicit __extended_grapheme_cluster_break(char32_t __first_code_point)
@@ -527,7 +527,7 @@ private:
 /// Therefore only this code point is extracted.
 template <class _CharT>
 class __extended_grapheme_cluster_view {
-  using _Iterator = typename basic_string_view<_CharT>::const_iterator;
+  using _Iterator _LIBCPP_NODEBUG = typename basic_string_view<_CharT>::const_iterator;
 
 public:
   _LIBCPP_HIDE_FROM_ABI constexpr explicit __extended_grapheme_cluster_view(_Iterator __first, _Iterator __last)
@@ -572,7 +572,7 @@ __extended_grapheme_cluster_view(_Iterator, _Iterator) -> __extended_grapheme_cl
 // This makes it easier to write code agnostic of the _LIBCPP_HAS_UNICODE define.
 template <class _CharT>
 class __code_point_view {
-  using _Iterator = typename basic_string_view<_CharT>::const_iterator;
+  using _Iterator _LIBCPP_NODEBUG = typename basic_string_view<_CharT>::const_iterator;
 
 public:
   _LIBCPP_HIDE_FROM_ABI constexpr explicit __code_point_view(_Iterator __first, _Iterator __last)
diff --git a/libcxx/include/__functional/binary_function.h b/libcxx/include/__functional/binary_function.h
index ddee3b170311..bde8b03ef828 100644
--- a/libcxx/include/__functional/binary_function.h
+++ b/libcxx/include/__functional/binary_function.h
@@ -42,11 +42,11 @@ struct __binary_function_keep_layout_base {
 _LIBCPP_DIAGNOSTIC_PUSH
 _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wdeprecated-declarations")
 template <class _Arg1, class _Arg2, class _Result>
-using __binary_function = binary_function<_Arg1, _Arg2, _Result>;
+using __binary_function _LIBCPP_NODEBUG = binary_function<_Arg1, _Arg2, _Result>;
 _LIBCPP_DIAGNOSTIC_POP
 #else
 template <class _Arg1, class _Arg2, class _Result>
-using __binary_function = __binary_function_keep_layout_base<_Arg1, _Arg2, _Result>;
+using __binary_function _LIBCPP_NODEBUG = __binary_function_keep_layout_base<_Arg1, _Arg2, _Result>;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/__functional/bind.h b/libcxx/include/__functional/bind.h
index f82c1517249b..e31ad2979035 100644
--- a/libcxx/include/__functional/bind.h
+++ b/libcxx/include/__functional/bind.h
@@ -198,7 +198,7 @@ __apply_functor(_Fp& __f, _BoundArgs& __bound_args, __tuple_indices<_Indx...>, _
 template <class _Fp, class... _BoundArgs>
 class __bind : public __weak_result_type<__decay_t<_Fp> > {
 protected:
-  using _Fd = __decay_t<_Fp>;
+  using _Fd _LIBCPP_NODEBUG = __decay_t<_Fp>;
   typedef tuple<__decay_t<_BoundArgs>...> _Td;
 
 private:
diff --git a/libcxx/include/__functional/boyer_moore_searcher.h b/libcxx/include/__functional/boyer_moore_searcher.h
index 52a58d57a8d4..1e49cc5464be 100644
--- a/libcxx/include/__functional/boyer_moore_searcher.h
+++ b/libcxx/include/__functional/boyer_moore_searcher.h
@@ -92,7 +92,7 @@ class _LIBCPP_TEMPLATE_VIS boyer_moore_searcher {
 private:
   using difference_type = typename std::iterator_traits<_RandomAccessIterator1>::difference_type;
   using value_type      = typename std::iterator_traits<_RandomAccessIterator1>::value_type;
-  using __skip_table_type =
+  using __skip_table_type _LIBCPP_NODEBUG =
       _BMSkipTable<value_type,
                    difference_type,
                    _Hash,
@@ -223,7 +223,7 @@ class _LIBCPP_TEMPLATE_VIS boyer_moore_horspool_searcher {
 private:
   using difference_type = typename iterator_traits<_RandomAccessIterator1>::difference_type;
   using value_type      = typename iterator_traits<_RandomAccessIterator1>::value_type;
-  using __skip_table_type =
+  using __skip_table_type _LIBCPP_NODEBUG =
       _BMSkipTable<value_type,
                    difference_type,
                    _Hash,
diff --git a/libcxx/include/__functional/function.h b/libcxx/include/__functional/function.h
index a421a3ef4f5f..b483e8ea8f85 100644
--- a/libcxx/include/__functional/function.h
+++ b/libcxx/include/__functional/function.h
@@ -576,7 +576,7 @@ private:
 // Used to choose between perfect forwarding or pass-by-value. Pass-by-value is
 // faster for types that can be passed in registers.
 template <typename _Tp>
-using __fast_forward = __conditional_t<is_scalar<_Tp>::value, _Tp, _Tp&&>;
+using __fast_forward _LIBCPP_NODEBUG = __conditional_t<is_scalar<_Tp>::value, _Tp, _Tp&&>;
 
 // __policy_invoker calls an instance of __alloc_func held in __policy_storage.
 
@@ -847,7 +847,7 @@ class _LIBCPP_TEMPLATE_VIS function<_Rp(_ArgTypes...)>
   };
 
   template <class _Fp>
-  using _EnableIfLValueCallable = __enable_if_t<__callable<_Fp&>::value>;
+  using _EnableIfLValueCallable _LIBCPP_NODEBUG = __enable_if_t<__callable<_Fp&>::value>;
 
 public:
   typedef _Rp result_type;
diff --git a/libcxx/include/__functional/perfect_forward.h b/libcxx/include/__functional/perfect_forward.h
index 8fd68db3d6eb..37c3d15b4bec 100644
--- a/libcxx/include/__functional/perfect_forward.h
+++ b/libcxx/include/__functional/perfect_forward.h
@@ -94,7 +94,7 @@ public:
 
 // __perfect_forward implements a perfect-forwarding call wrapper as explained in [func.require].
 template <class _Op, class... _Args>
-using __perfect_forward = __perfect_forward_impl<_Op, index_sequence_for<_Args...>, _Args...>;
+using __perfect_forward _LIBCPP_NODEBUG = __perfect_forward_impl<_Op, index_sequence_for<_Args...>, _Args...>;
 
 #endif // _LIBCPP_STD_VER >= 17
 
diff --git a/libcxx/include/__functional/unary_function.h b/libcxx/include/__functional/unary_function.h
index 69b1bc94220a..769ffc9893a7 100644
--- a/libcxx/include/__functional/unary_function.h
+++ b/libcxx/include/__functional/unary_function.h
@@ -39,11 +39,11 @@ struct __unary_function_keep_layout_base {
 _LIBCPP_DIAGNOSTIC_PUSH
 _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wdeprecated-declarations")
 template <class _Arg, class _Result>
-using __unary_function = unary_function<_Arg, _Result>;
+using __unary_function _LIBCPP_NODEBUG = unary_function<_Arg, _Result>;
 _LIBCPP_DIAGNOSTIC_POP
 #else
 template <class _Arg, class _Result>
-using __unary_function = __unary_function_keep_layout_base<_Arg, _Result>;
+using __unary_function _LIBCPP_NODEBUG = __unary_function_keep_layout_base<_Arg, _Result>;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/__hash_table b/libcxx/include/__hash_table
index 9c821ea69081..7788f687746f 100644
--- a/libcxx/include/__hash_table
+++ b/libcxx/include/__hash_table
@@ -111,8 +111,8 @@ struct __hash_node_base {
 template <class _Tp, class _VoidPtr>
 struct __hash_node : public __hash_node_base< __rebind_pointer_t<_VoidPtr, __hash_node<_Tp, _VoidPtr> > > {
   typedef _Tp __node_value_type;
-  using _Base          = __hash_node_base<__rebind_pointer_t<_VoidPtr, __hash_node<_Tp, _VoidPtr> > >;
-  using __next_pointer = typename _Base::__next_pointer;
+  using _Base _LIBCPP_NODEBUG          = __hash_node_base<__rebind_pointer_t<_VoidPtr, __hash_node<_Tp, _VoidPtr> > >;
+  using __next_pointer _LIBCPP_NODEBUG = typename _Base::__next_pointer;
 
   size_t __hash_;
 
diff --git a/libcxx/include/__iterator/aliasing_iterator.h b/libcxx/include/__iterator/aliasing_iterator.h
index aeb5b4a88ec3..e01127142ae9 100644
--- a/libcxx/include/__iterator/aliasing_iterator.h
+++ b/libcxx/include/__iterator/aliasing_iterator.h
@@ -31,8 +31,8 @@ struct __aliasing_iterator_wrapper {
   class __iterator {
     _BaseIter __base_ = nullptr;
 
-    using __iter_traits     = iterator_traits<_BaseIter>;
-    using __base_value_type = typename __iter_traits::value_type;
+    using __iter_traits _LIBCPP_NODEBUG     = iterator_traits<_BaseIter>;
+    using __base_value_type _LIBCPP_NODEBUG = typename __iter_traits::value_type;
 
     static_assert(__has_random_access_iterator_category<_BaseIter>::value,
                   "The base iterator has to be a random access iterator!");
@@ -120,7 +120,7 @@ struct __aliasing_iterator_wrapper {
 
 // This is required to avoid ADL instantiations on _BaseT
 template <class _BaseT, class _Alias>
-using __aliasing_iterator = typename __aliasing_iterator_wrapper<_BaseT, _Alias>::__iterator;
+using __aliasing_iterator _LIBCPP_NODEBUG = typename __aliasing_iterator_wrapper<_BaseT, _Alias>::__iterator;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__iterator/concepts.h b/libcxx/include/__iterator/concepts.h
index 1c227933a482..6e5ac1d3af37 100644
--- a/libcxx/include/__iterator/concepts.h
+++ b/libcxx/include/__iterator/concepts.h
@@ -67,10 +67,10 @@ template <class _In>
 concept indirectly_readable = __indirectly_readable_impl<remove_cvref_t<_In>>;
 
 template <class _Tp>
-using __projected_iterator_t = typename _Tp::__projected_iterator;
+using __projected_iterator_t _LIBCPP_NODEBUG = typename _Tp::__projected_iterator;
 
 template <class _Tp>
-using __projected_projection_t = typename _Tp::__projected_projection;
+using __projected_projection_t _LIBCPP_NODEBUG = typename _Tp::__projected_projection;
 
 template <class _Tp>
 concept __specialization_of_projected = requires {
@@ -89,7 +89,7 @@ struct __indirect_value_t_impl<_Tp> {
 };
 
 template <indirectly_readable _Tp>
-using __indirect_value_t = typename __indirect_value_t_impl<_Tp>::type;
+using __indirect_value_t _LIBCPP_NODEBUG = typename __indirect_value_t_impl<_Tp>::type;
 
 template <indirectly_readable _Tp>
 using iter_common_reference_t = common_reference_t<iter_reference_t<_Tp>, __indirect_value_t<_Tp>>;
@@ -274,7 +274,7 @@ concept indirectly_copyable_storable =
 #endif // _LIBCPP_STD_VER >= 20
 
 template <class _Tp>
-using __has_random_access_iterator_category_or_concept
+using __has_random_access_iterator_category_or_concept _LIBCPP_NODEBUG
 #if _LIBCPP_STD_VER >= 20
     = integral_constant<bool, random_access_iterator<_Tp>>;
 #else  // _LIBCPP_STD_VER < 20
diff --git a/libcxx/include/__iterator/insert_iterator.h b/libcxx/include/__iterator/insert_iterator.h
index b3311042014f..e0ee0ce035e2 100644
--- a/libcxx/include/__iterator/insert_iterator.h
+++ b/libcxx/include/__iterator/insert_iterator.h
@@ -29,10 +29,10 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 20
 template <class _Container>
-using __insert_iterator_iter_t = ranges::iterator_t<_Container>;
+using __insert_iterator_iter_t _LIBCPP_NODEBUG = ranges::iterator_t<_Container>;
 #else
 template <class _Container>
-using __insert_iterator_iter_t = typename _Container::iterator;
+using __insert_iterator_iter_t _LIBCPP_NODEBUG = typename _Container::iterator;
 #endif
 
 _LIBCPP_SUPPRESS_DEPRECATED_PUSH
diff --git a/libcxx/include/__iterator/iterator_traits.h b/libcxx/include/__iterator/iterator_traits.h
index eb6ba8b62fb3..db68dd2c377a 100644
--- a/libcxx/include/__iterator/iterator_traits.h
+++ b/libcxx/include/__iterator/iterator_traits.h
@@ -47,7 +47,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #if _LIBCPP_STD_VER >= 20
 
 template <class _Tp>
-using __with_reference = _Tp&;
+using __with_reference _LIBCPP_NODEBUG = _Tp&;
 
 template <class _Tp>
 concept __can_reference = requires { typename __with_reference<_Tp>; };
@@ -80,19 +80,20 @@ struct __iter_traits_cache {
   using type = _If< __is_primary_template<iterator_traits<_Iter> >::value, _Iter, iterator_traits<_Iter> >;
 };
 template <class _Iter>
-using _ITER_TRAITS = typename __iter_traits_cache<_Iter>::type;
+using _ITER_TRAITS _LIBCPP_NODEBUG = typename __iter_traits_cache<_Iter>::type;
 
 struct __iter_concept_concept_test {
   template <class _Iter>
-  using _Apply = typename _ITER_TRAITS<_Iter>::iterator_concept;
+  using _Apply _LIBCPP_NODEBUG = typename _ITER_TRAITS<_Iter>::iterator_concept;
 };
 struct __iter_concept_category_test {
   template <class _Iter>
-  using _Apply = typename _ITER_TRAITS<_Iter>::iterator_category;
+  using _Apply _LIBCPP_NODEBUG = typename _ITER_TRAITS<_Iter>::iterator_category;
 };
 struct __iter_concept_random_fallback {
   template <class _Iter>
-  using _Apply = __enable_if_t< __is_primary_template<iterator_traits<_Iter> >::value, random_access_iterator_tag >;
+  using _Apply _LIBCPP_NODEBUG =
+      __enable_if_t<__is_primary_template<iterator_traits<_Iter> >::value, random_access_iterator_tag>;
 };
 
 template <class _Iter, class _Tester>
@@ -106,7 +107,7 @@ struct __iter_concept_cache {
 };
 
 template <class _Iter>
-using _ITER_CONCEPT = typename __iter_concept_cache<_Iter>::type::template _Apply<_Iter>;
+using _ITER_CONCEPT _LIBCPP_NODEBUG = typename __iter_concept_cache<_Iter>::type::template _Apply<_Iter>;
 
 template <class _Tp>
 struct __has_iterator_typedefs {
@@ -364,7 +365,7 @@ struct __iterator_traits<_Ip> {
 
 template <class _Ip>
 struct iterator_traits : __iterator_traits<_Ip> {
-  using __primary_template = iterator_traits;
+  using __primary_template _LIBCPP_NODEBUG = iterator_traits;
 };
 
 #else  // _LIBCPP_STD_VER >= 20
@@ -397,7 +398,7 @@ struct __iterator_traits<_Iter, true>
 
 template <class _Iter>
 struct _LIBCPP_TEMPLATE_VIS iterator_traits : __iterator_traits<_Iter, __has_iterator_typedefs<_Iter>::value> {
-  using __primary_template = iterator_traits;
+  using __primary_template _LIBCPP_NODEBUG = iterator_traits;
 };
 #endif // _LIBCPP_STD_VER >= 20
 
@@ -430,16 +431,19 @@ template <class _Tp, class _Up>
 struct __has_iterator_concept_convertible_to<_Tp, _Up, false> : false_type {};
 
 template <class _Tp>
-using __has_input_iterator_category = __has_iterator_category_convertible_to<_Tp, input_iterator_tag>;
+using __has_input_iterator_category _LIBCPP_NODEBUG = __has_iterator_category_convertible_to<_Tp, input_iterator_tag>;
 
 template <class _Tp>
-using __has_forward_iterator_category = __has_iterator_category_convertible_to<_Tp, forward_iterator_tag>;
+using __has_forward_iterator_category _LIBCPP_NODEBUG =
+    __has_iterator_category_convertible_to<_Tp, forward_iterator_tag>;
 
 template <class _Tp>
-using __has_bidirectional_iterator_category = __has_iterator_category_convertible_to<_Tp, bidirectional_iterator_tag>;
+using __has_bidirectional_iterator_category _LIBCPP_NODEBUG =
+    __has_iterator_category_convertible_to<_Tp, bidirectional_iterator_tag>;
 
 template <class _Tp>
-using __has_random_access_iterator_category = __has_iterator_category_convertible_to<_Tp, random_access_iterator_tag>;
+using __has_random_access_iterator_category _LIBCPP_NODEBUG =
+    __has_iterator_category_convertible_to<_Tp, random_access_iterator_tag>;
 
 // __libcpp_is_contiguous_iterator determines if an iterator is known by
 // libc++ to be contiguous, either because it advertises itself as such
@@ -466,48 +470,49 @@ template <class _Iter>
 class __wrap_iter;
 
 template <class _Tp>
-using __has_exactly_input_iterator_category =
+using __has_exactly_input_iterator_category _LIBCPP_NODEBUG =
     integral_constant<bool,
                       __has_iterator_category_convertible_to<_Tp, input_iterator_tag>::value &&
                           !__has_iterator_category_convertible_to<_Tp, forward_iterator_tag>::value>;
 
 template <class _Tp>
-using __has_exactly_forward_iterator_category =
+using __has_exactly_forward_iterator_category _LIBCPP_NODEBUG =
     integral_constant<bool,
                       __has_iterator_category_convertible_to<_Tp, forward_iterator_tag>::value &&
                           !__has_iterator_category_convertible_to<_Tp, bidirectional_iterator_tag>::value>;
 
 template <class _Tp>
-using __has_exactly_bidirectional_iterator_category =
+using __has_exactly_bidirectional_iterator_category _LIBCPP_NODEBUG =
     integral_constant<bool,
                       __has_iterator_category_convertible_to<_Tp, bidirectional_iterator_tag>::value &&
                           !__has_iterator_category_convertible_to<_Tp, random_access_iterator_tag>::value>;
 
 template <class _InputIterator>
-using __iter_value_type = typename iterator_traits<_InputIterator>::value_type;
+using __iter_value_type _LIBCPP_NODEBUG = typename iterator_traits<_InputIterator>::value_type;
 
 template <class _InputIterator>
-using __iter_key_type = __remove_const_t<typename iterator_traits<_InputIterator>::value_type::first_type>;
+using __iter_key_type _LIBCPP_NODEBUG =
+    __remove_const_t<typename iterator_traits<_InputIterator>::value_type::first_type>;
 
 template <class _InputIterator>
-using __iter_mapped_type = typename iterator_traits<_InputIterator>::value_type::second_type;
+using __iter_mapped_type _LIBCPP_NODEBUG = typename iterator_traits<_InputIterator>::value_type::second_type;
 
 template <class _InputIterator>
-using __iter_to_alloc_type =
+using __iter_to_alloc_type _LIBCPP_NODEBUG =
     pair<const typename iterator_traits<_InputIterator>::value_type::first_type,
          typename iterator_traits<_InputIterator>::value_type::second_type>;
 
 template <class _Iter>
-using __iterator_category_type = typename iterator_traits<_Iter>::iterator_category;
+using __iterator_category_type _LIBCPP_NODEBUG = typename iterator_traits<_Iter>::iterator_category;
 
 template <class _Iter>
-using __iterator_pointer_type = typename iterator_traits<_Iter>::pointer;
+using __iterator_pointer_type _LIBCPP_NODEBUG = typename iterator_traits<_Iter>::pointer;
 
 template <class _Iter>
-using __iter_diff_t = typename iterator_traits<_Iter>::difference_type;
+using __iter_diff_t _LIBCPP_NODEBUG = typename iterator_traits<_Iter>::difference_type;
 
 template <class _Iter>
-using __iter_reference = typename iterator_traits<_Iter>::reference;
+using __iter_reference _LIBCPP_NODEBUG = typename iterator_traits<_Iter>::reference;
 
 #if _LIBCPP_STD_VER >= 20
 
diff --git a/libcxx/include/__iterator/projected.h b/libcxx/include/__iterator/projected.h
index 1c560ec05500..d12f0167de1d 100644
--- a/libcxx/include/__iterator/projected.h
+++ b/libcxx/include/__iterator/projected.h
@@ -26,9 +26,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _It, class _Proj>
 struct __projected_impl {
   struct __type {
-    using __primary_template     = __type;
-    using __projected_iterator   = _It;
-    using __projected_projection = _Proj;
+    using __primary_template _LIBCPP_NODEBUG     = __type;
+    using __projected_iterator _LIBCPP_NODEBUG   = _It;
+    using __projected_projection _LIBCPP_NODEBUG = _Proj;
 
     using value_type = remove_cvref_t<indirect_result_t<_Proj&, _It>>;
     indirect_result_t<_Proj&, _It> operator*() const; // not defined
@@ -38,9 +38,9 @@ struct __projected_impl {
 template <weakly_incrementable _It, class _Proj>
 struct __projected_impl<_It, _Proj> {
   struct __type {
-    using __primary_template     = __type;
-    using __projected_iterator   = _It;
-    using __projected_projection = _Proj;
+    using __primary_template _LIBCPP_NODEBUG     = __type;
+    using __projected_iterator _LIBCPP_NODEBUG   = _It;
+    using __projected_projection _LIBCPP_NODEBUG = _Proj;
 
     using value_type      = remove_cvref_t<indirect_result_t<_Proj&, _It>>;
     using difference_type = iter_difference_t<_It>;
diff --git a/libcxx/include/__iterator/ranges_iterator_traits.h b/libcxx/include/__iterator/ranges_iterator_traits.h
index 859e7082048a..9a31b651eb5d 100644
--- a/libcxx/include/__iterator/ranges_iterator_traits.h
+++ b/libcxx/include/__iterator/ranges_iterator_traits.h
@@ -24,13 +24,13 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #if _LIBCPP_STD_VER >= 23
 
 template <ranges::input_range _Range>
-using __range_key_type = __remove_const_t<typename ranges::range_value_t<_Range>::first_type>;
+using __range_key_type _LIBCPP_NODEBUG = __remove_const_t<typename ranges::range_value_t<_Range>::first_type>;
 
 template <ranges::input_range _Range>
-using __range_mapped_type = typename ranges::range_value_t<_Range>::second_type;
+using __range_mapped_type _LIBCPP_NODEBUG = typename ranges::range_value_t<_Range>::second_type;
 
 template <ranges::input_range _Range>
-using __range_to_alloc_type =
+using __range_to_alloc_type _LIBCPP_NODEBUG =
     pair<const typename ranges::range_value_t<_Range>::first_type, typename ranges::range_value_t<_Range>::second_type>;
 
 #endif
diff --git a/libcxx/include/__iterator/reverse_iterator.h b/libcxx/include/__iterator/reverse_iterator.h
index 5e88d86ad5e9..5bd1f868d3ff 100644
--- a/libcxx/include/__iterator/reverse_iterator.h
+++ b/libcxx/include/__iterator/reverse_iterator.h
@@ -329,8 +329,8 @@ __reverse_range(_Range&& __range) {
 
 template <class _Iter, bool __b>
 struct __unwrap_iter_impl<reverse_iterator<reverse_iterator<_Iter> >, __b> {
-  using _UnwrappedIter  = decltype(__unwrap_iter_impl<_Iter>::__unwrap(std::declval<_Iter>()));
-  using _ReverseWrapper = reverse_iterator<reverse_iterator<_Iter> >;
+  using _UnwrappedIter _LIBCPP_NODEBUG  = decltype(__unwrap_iter_impl<_Iter>::__unwrap(std::declval<_Iter>()));
+  using _ReverseWrapper _LIBCPP_NODEBUG = reverse_iterator<reverse_iterator<_Iter> >;
 
   static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _ReverseWrapper
   __rewrap(_ReverseWrapper __orig_iter, _UnwrappedIter __unwrapped_iter) {
diff --git a/libcxx/include/__iterator/segmented_iterator.h b/libcxx/include/__iterator/segmented_iterator.h
index 8cb54a35a7f5..7a8e1addeacd 100644
--- a/libcxx/include/__iterator/segmented_iterator.h
+++ b/libcxx/include/__iterator/segmented_iterator.h
@@ -72,7 +72,7 @@ template <class _Tp>
 struct __has_specialization<_Tp, sizeof(_Tp) * 0> : true_type {};
 
 template <class _Iterator>
-using __is_segmented_iterator = __has_specialization<__segmented_iterator_traits<_Iterator> >;
+using __is_segmented_iterator _LIBCPP_NODEBUG = __has_specialization<__segmented_iterator_traits<_Iterator> >;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__locale b/libcxx/include/__locale
index b675e01bac81..94dc8a08437b 100644
--- a/libcxx/include/__locale
+++ b/libcxx/include/__locale
@@ -50,7 +50,7 @@ _LIBCPP_HIDE_FROM_ABI const _Facet& use_facet(const locale&);
 class _LIBCPP_EXPORTED_FROM_ABI locale {
 public:
   // locale is essentially a shared_ptr that doesn't support weak_ptrs and never got a move constructor.
-  using __trivially_relocatable = locale;
+  using __trivially_relocatable _LIBCPP_NODEBUG = locale;
 
   // types:
   class _LIBCPP_EXPORTED_FROM_ABI facet;
diff --git a/libcxx/include/__locale_dir/locale_base_api.h b/libcxx/include/__locale_dir/locale_base_api.h
index c8097beb9052..9957429c1e7c 100644
--- a/libcxx/include/__locale_dir/locale_base_api.h
+++ b/libcxx/include/__locale_dir/locale_base_api.h
@@ -135,7 +135,7 @@ namespace __locale {
 //
 // Locale management
 //
-using __locale_t = locale_t;
+using __locale_t _LIBCPP_NODEBUG = locale_t;
 
 inline _LIBCPP_HIDE_FROM_ABI __locale_t __newlocale(int __category_mask, const char* __name, __locale_t __loc) {
   return newlocale(__category_mask, __name, __loc);
diff --git a/libcxx/include/__mdspan/extents.h b/libcxx/include/__mdspan/extents.h
index edbc30a7a40e..65a697769bda 100644
--- a/libcxx/include/__mdspan/extents.h
+++ b/libcxx/include/__mdspan/extents.h
@@ -129,14 +129,14 @@ private:
   // Static values member
   static constexpr size_t __size_         = sizeof...(_Values);
   static constexpr size_t __size_dynamic_ = ((_Values == _DynTag) + ... + 0);
-  using _StaticValues                     = __static_array<_TStatic, _Values...>;
-  using _DynamicValues                    = __possibly_empty_array<_TDynamic, __size_dynamic_>;
+  using _StaticValues _LIBCPP_NODEBUG     = __static_array<_TStatic, _Values...>;
+  using _DynamicValues _LIBCPP_NODEBUG    = __possibly_empty_array<_TDynamic, __size_dynamic_>;
 
   // Dynamic values member
   _LIBCPP_NO_UNIQUE_ADDRESS _DynamicValues __dyn_vals_;
 
   // static mapping of indices to the position in the dynamic values array
-  using _DynamicIdxMap = __static_partial_sums<static_cast<size_t>(_Values == _DynTag)...>;
+  using _DynamicIdxMap _LIBCPP_NODEBUG = __static_partial_sums<static_cast<size_t>(_Values == _DynTag)...>;
 
   template <size_t... _Indices>
   _LIBCPP_HIDE_FROM_ABI static constexpr _DynamicValues __zeros(index_sequence<_Indices...>) noexcept {
@@ -292,7 +292,8 @@ private:
   static constexpr rank_type __rank_dynamic_ = ((_Extents == dynamic_extent) + ... + 0);
 
   // internal storage type using __maybe_static_array
-  using _Values = __mdspan_detail::__maybe_static_array<_IndexType, size_t, dynamic_extent, _Extents...>;
+  using _Values _LIBCPP_NODEBUG =
+      __mdspan_detail::__maybe_static_array<_IndexType, size_t, dynamic_extent, _Extents...>;
   [[no_unique_address]] _Values __vals_;
 
 public:
diff --git a/libcxx/include/__memory/allocation_guard.h b/libcxx/include/__memory/allocation_guard.h
index 66d6a5002c29..66edcd92ed61 100644
--- a/libcxx/include/__memory/allocation_guard.h
+++ b/libcxx/include/__memory/allocation_guard.h
@@ -45,8 +45,8 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 // custom allocator.
 template <class _Alloc>
 struct __allocation_guard {
-  using _Pointer = typename allocator_traits<_Alloc>::pointer;
-  using _Size    = typename allocator_traits<_Alloc>::size_type;
+  using _Pointer _LIBCPP_NODEBUG = typename allocator_traits<_Alloc>::pointer;
+  using _Size _LIBCPP_NODEBUG    = typename allocator_traits<_Alloc>::size_type;
 
   template <class _AllocT> // we perform the allocator conversion inside the constructor
   _LIBCPP_HIDE_FROM_ABI explicit __allocation_guard(_AllocT __alloc, _Size __n)
diff --git a/libcxx/include/__memory/pointer_traits.h b/libcxx/include/__memory/pointer_traits.h
index e35cfb7c3b87..afe3d1bf8a2d 100644
--- a/libcxx/include/__memory/pointer_traits.h
+++ b/libcxx/include/__memory/pointer_traits.h
@@ -176,10 +176,10 @@ public:
 
 #ifndef _LIBCPP_CXX03_LANG
 template <class _From, class _To>
-using __rebind_pointer_t = typename pointer_traits<_From>::template rebind<_To>;
+using __rebind_pointer_t _LIBCPP_NODEBUG = typename pointer_traits<_From>::template rebind<_To>;
 #else
 template <class _From, class _To>
-using __rebind_pointer_t = typename pointer_traits<_From>::template rebind<_To>::other;
+using __rebind_pointer_t _LIBCPP_NODEBUG = typename pointer_traits<_From>::template rebind<_To>::other;
 #endif
 
 // to_address
@@ -276,7 +276,7 @@ struct __pointer_of<_Tp> {
 };
 
 template <typename _Tp>
-using __pointer_of_t = typename __pointer_of<_Tp>::type;
+using __pointer_of_t _LIBCPP_NODEBUG = typename __pointer_of<_Tp>::type;
 
 template <class _Tp, class _Up>
 struct __pointer_of_or {
@@ -290,7 +290,7 @@ struct __pointer_of_or<_Tp, _Up> {
 };
 
 template <typename _Tp, typename _Up>
-using __pointer_of_or_t = typename __pointer_of_or<_Tp, _Up>::type;
+using __pointer_of_or_t _LIBCPP_NODEBUG = typename __pointer_of_or<_Tp, _Up>::type;
 
 template <class _Smart>
 concept __resettable_smart_pointer = requires(_Smart __s) { __s.reset(); };
diff --git a/libcxx/include/__memory/shared_ptr.h b/libcxx/include/__memory/shared_ptr.h
index 97e4031499ed..06b1fc488cf5 100644
--- a/libcxx/include/__memory/shared_ptr.h
+++ b/libcxx/include/__memory/shared_ptr.h
@@ -141,7 +141,7 @@ struct __for_overwrite_tag {};
 
 template <class _Tp, class _Alloc>
 struct __shared_ptr_emplace : __shared_weak_count {
-  using __value_type = __remove_cv_t<_Tp>;
+  using __value_type _LIBCPP_NODEBUG = __remove_cv_t<_Tp>;
 
   template <class... _Args,
             class _Allocator                                                                         = _Alloc,
@@ -293,7 +293,8 @@ struct __shared_ptr_deleter_ctor_reqs {
 };
 
 template <class _Dp>
-using __shared_ptr_nullptr_deleter_ctor_reqs = _And<is_move_constructible<_Dp>, __well_formed_deleter<_Dp, nullptr_t> >;
+using __shared_ptr_nullptr_deleter_ctor_reqs _LIBCPP_NODEBUG =
+    _And<is_move_constructible<_Dp>, __well_formed_deleter<_Dp, nullptr_t> >;
 
 #if defined(_LIBCPP_ABI_ENABLE_SHARED_PTR_TRIVIAL_ABI)
 #  define _LIBCPP_SHARED_PTR_TRIVIAL_ABI __attribute__((__trivial_abi__))
@@ -315,7 +316,7 @@ public:
 
   // A shared_ptr contains only two raw pointers which point to the heap and move constructing already doesn't require
   // any bookkeeping, so it's always trivially relocatable.
-  using __trivially_relocatable = shared_ptr;
+  using __trivially_relocatable _LIBCPP_NODEBUG = shared_ptr;
 
 private:
   element_type* __ptr_;
@@ -1210,7 +1211,7 @@ public:
 
   // A weak_ptr contains only two raw pointers which point to the heap and move constructing already doesn't require
   // any bookkeeping, so it's always trivially relocatable.
-  using __trivially_relocatable = weak_ptr;
+  using __trivially_relocatable _LIBCPP_NODEBUG = weak_ptr;
 
 private:
   element_type* __ptr_;
diff --git a/libcxx/include/__memory/unique_ptr.h b/libcxx/include/__memory/unique_ptr.h
index 2368f7b03e00..29d391fc80fe 100644
--- a/libcxx/include/__memory/unique_ptr.h
+++ b/libcxx/include/__memory/unique_ptr.h
@@ -153,7 +153,7 @@ public:
   //
   // This unique_ptr implementation only contains a pointer to the unique object and a deleter, so there are no
   // references to itself. This means that the entire structure is trivially relocatable if its members are.
-  using __trivially_relocatable = __conditional_t<
+  using __trivially_relocatable _LIBCPP_NODEBUG = __conditional_t<
       __libcpp_is_trivially_relocatable<pointer>::value && __libcpp_is_trivially_relocatable<deleter_type>::value,
       unique_ptr,
       void>;
@@ -189,7 +189,7 @@ private:
                      (!is_reference<_Dp>::value && is_convertible<_UDel, _Dp>::value) >;
 
   template <class _UDel>
-  using _EnableIfDeleterAssignable = __enable_if_t< is_assignable<_Dp&, _UDel&&>::value >;
+  using _EnableIfDeleterAssignable _LIBCPP_NODEBUG = __enable_if_t< is_assignable<_Dp&, _UDel&&>::value >;
 
 public:
   template <bool _Dummy = true, class = _EnableIfDeleterDefaultConstructible<_Dummy> >
@@ -419,7 +419,7 @@ public:
   //
   // This unique_ptr implementation only contains a pointer to the unique object and a deleter, so there are no
   // references to itself. This means that the entire structure is trivially relocatable if its members are.
-  using __trivially_relocatable = __conditional_t<
+  using __trivially_relocatable _LIBCPP_NODEBUG = __conditional_t<
       __libcpp_is_trivially_relocatable<pointer>::value && __libcpp_is_trivially_relocatable<deleter_type>::value,
       unique_ptr,
       void>;
@@ -430,9 +430,9 @@ private:
 
   _LIBCPP_COMPRESSED_PAIR(pointer, __ptr_, deleter_type, __deleter_);
 #ifdef _LIBCPP_ABI_BOUNDED_UNIQUE_PTR
-  using _BoundsChecker = __unique_ptr_array_bounds_stored;
+  using _BoundsChecker _LIBCPP_NODEBUG = __unique_ptr_array_bounds_stored;
 #else
-  using _BoundsChecker = __unique_ptr_array_bounds_stateless;
+  using _BoundsChecker _LIBCPP_NODEBUG = __unique_ptr_array_bounds_stateless;
 #endif
   _LIBCPP_NO_UNIQUE_ADDRESS _BoundsChecker __checker_;
 
diff --git a/libcxx/include/__memory/unique_temporary_buffer.h b/libcxx/include/__memory/unique_temporary_buffer.h
index ca6292338c00..dea7fa8e1872 100644
--- a/libcxx/include/__memory/unique_temporary_buffer.h
+++ b/libcxx/include/__memory/unique_temporary_buffer.h
@@ -45,7 +45,7 @@ struct __temporary_buffer_deleter {
 };
 
 template <class _Tp>
-using __unique_temporary_buffer = unique_ptr<_Tp, __temporary_buffer_deleter<_Tp> >;
+using __unique_temporary_buffer _LIBCPP_NODEBUG = unique_ptr<_Tp, __temporary_buffer_deleter<_Tp> >;
 
 template <class _Tp>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_NO_CFI _LIBCPP_CONSTEXPR_SINCE_CXX23 __unique_temporary_buffer<_Tp>
diff --git a/libcxx/include/__node_handle b/libcxx/include/__node_handle
index d0b35bfd1934..8f32f2de8339 100644
--- a/libcxx/include/__node_handle
+++ b/libcxx/include/__node_handle
@@ -188,10 +188,10 @@ struct __map_node_handle_specifics {
 };
 
 template <class _NodeType, class _Alloc>
-using __set_node_handle = __basic_node_handle< _NodeType, _Alloc, __set_node_handle_specifics>;
+using __set_node_handle _LIBCPP_NODEBUG = __basic_node_handle< _NodeType, _Alloc, __set_node_handle_specifics>;
 
 template <class _NodeType, class _Alloc>
-using __map_node_handle = __basic_node_handle< _NodeType, _Alloc, __map_node_handle_specifics>;
+using __map_node_handle _LIBCPP_NODEBUG = __basic_node_handle< _NodeType, _Alloc, __map_node_handle_specifics>;
 
 template <class _Iterator, class _NodeType>
 struct _LIBCPP_TEMPLATE_VIS __insert_return_type {
diff --git a/libcxx/include/__ostream/basic_ostream.h b/libcxx/include/__ostream/basic_ostream.h
index cf4d26167aeb..97226476e5ef 100644
--- a/libcxx/include/__ostream/basic_ostream.h
+++ b/libcxx/include/__ostream/basic_ostream.h
@@ -88,6 +88,55 @@ public:
     return *this;
   }
 
+  template <class _Tp>
+  _LIBCPP_HIDE_FROM_ABI basic_ostream& __put_num(_Tp __value) {
+#  if _LIBCPP_HAS_EXCEPTIONS
+    try {
+#  endif // _LIBCPP_HAS_EXCEPTIONS
+      sentry __s(*this);
+      if (__s) {
+        using _Fp          = num_put<char_type, ostreambuf_iterator<char_type, traits_type> >;
+        const _Fp& __facet = std::use_facet<_Fp>(this->getloc());
+        if (__facet.put(*this, *this, this->fill(), __value).failed())
+          this->setstate(ios_base::badbit | ios_base::failbit);
+      }
+#  if _LIBCPP_HAS_EXCEPTIONS
+    } catch (...) {
+      this->__set_badbit_and_consider_rethrow();
+    }
+#  endif // _LIBCPP_HAS_EXCEPTIONS
+    return *this;
+  }
+
+  template <class _Tp>
+  _LIBCPP_HIDE_FROM_ABI basic_ostream& __put_num_integer_promote(_Tp __value) {
+#  if _LIBCPP_HAS_EXCEPTIONS
+    try {
+#  endif // _LIBCPP_HAS_EXCEPTIONS
+      sentry __s(*this);
+      if (__s) {
+        ios_base::fmtflags __flags = ios_base::flags() & ios_base::basefield;
+
+        using _Fp          = num_put<char_type, ostreambuf_iterator<char_type, traits_type> >;
+        const _Fp& __facet = std::use_facet<_Fp>(this->getloc());
+        if (__facet
+                .put(*this,
+                     *this,
+                     this->fill(),
+                     __flags == ios_base::oct || __flags == ios_base::hex
+                         ? static_cast<__copy_unsigned_t<_Tp, long> >(std::__to_unsigned_like(__value))
+                         : static_cast<__copy_unsigned_t<_Tp, long> >(__value))
+                .failed())
+          this->setstate(ios_base::badbit | ios_base::failbit);
+      }
+#  if _LIBCPP_HAS_EXCEPTIONS
+    } catch (...) {
+      this->__set_badbit_and_consider_rethrow();
+    }
+#  endif // _LIBCPP_HAS_EXCEPTIONS
+    return *this;
+  }
+
   basic_ostream& operator<<(bool __n);
   basic_ostream& operator<<(short __n);
   basic_ostream& operator<<(unsigned short __n);
@@ -225,276 +274,67 @@ basic_ostream<_CharT, _Traits>::operator<<(basic_streambuf<char_type, traits_typ
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::operator<<(bool __n) {
-#  if _LIBCPP_HAS_EXCEPTIONS
-  try {
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-    sentry __s(*this);
-    if (__s) {
-      typedef num_put<char_type, ostreambuf_iterator<char_type, traits_type> > _Fp;
-      const _Fp& __f = std::use_facet<_Fp>(this->getloc());
-      if (__f.put(*this, *this, this->fill(), __n).failed())
-        this->setstate(ios_base::badbit | ios_base::failbit);
-    }
-#  if _LIBCPP_HAS_EXCEPTIONS
-  } catch (...) {
-    this->__set_badbit_and_consider_rethrow();
-  }
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-  return *this;
+  return __put_num(__n);
 }
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::operator<<(short __n) {
-#  if _LIBCPP_HAS_EXCEPTIONS
-  try {
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-    sentry __s(*this);
-    if (__s) {
-      ios_base::fmtflags __flags = ios_base::flags() & ios_base::basefield;
-      typedef num_put<char_type, ostreambuf_iterator<char_type, traits_type> > _Fp;
-      const _Fp& __f = std::use_facet<_Fp>(this->getloc());
-      if (__f.put(*this,
-                  *this,
-                  this->fill(),
-                  __flags == ios_base::oct || __flags == ios_base::hex
-                      ? static_cast<long>(static_cast<unsigned short>(__n))
-                      : static_cast<long>(__n))
-              .failed())
-        this->setstate(ios_base::badbit | ios_base::failbit);
-    }
-#  if _LIBCPP_HAS_EXCEPTIONS
-  } catch (...) {
-    this->__set_badbit_and_consider_rethrow();
-  }
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-  return *this;
+  return __put_num_integer_promote(__n);
 }
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::operator<<(unsigned short __n) {
-#  if _LIBCPP_HAS_EXCEPTIONS
-  try {
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-    sentry __s(*this);
-    if (__s) {
-      typedef num_put<char_type, ostreambuf_iterator<char_type, traits_type> > _Fp;
-      const _Fp& __f = std::use_facet<_Fp>(this->getloc());
-      if (__f.put(*this, *this, this->fill(), static_cast<unsigned long>(__n)).failed())
-        this->setstate(ios_base::badbit | ios_base::failbit);
-    }
-#  if _LIBCPP_HAS_EXCEPTIONS
-  } catch (...) {
-    this->__set_badbit_and_consider_rethrow();
-  }
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-  return *this;
+  return __put_num_integer_promote(__n);
 }
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::operator<<(int __n) {
-#  if _LIBCPP_HAS_EXCEPTIONS
-  try {
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-    sentry __s(*this);
-    if (__s) {
-      ios_base::fmtflags __flags = ios_base::flags() & ios_base::basefield;
-      typedef num_put<char_type, ostreambuf_iterator<char_type, traits_type> > _Fp;
-      const _Fp& __f = std::use_facet<_Fp>(this->getloc());
-      if (__f.put(*this,
-                  *this,
-                  this->fill(),
-                  __flags == ios_base::oct || __flags == ios_base::hex
-                      ? static_cast<long>(static_cast<unsigned int>(__n))
-                      : static_cast<long>(__n))
-              .failed())
-        this->setstate(ios_base::badbit | ios_base::failbit);
-    }
-#  if _LIBCPP_HAS_EXCEPTIONS
-  } catch (...) {
-    this->__set_badbit_and_consider_rethrow();
-  }
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-  return *this;
+  return __put_num_integer_promote(__n);
 }
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::operator<<(unsigned int __n) {
-#  if _LIBCPP_HAS_EXCEPTIONS
-  try {
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-    sentry __s(*this);
-    if (__s) {
-      typedef num_put<char_type, ostreambuf_iterator<char_type, traits_type> > _Fp;
-      const _Fp& __f = std::use_facet<_Fp>(this->getloc());
-      if (__f.put(*this, *this, this->fill(), static_cast<unsigned long>(__n)).failed())
-        this->setstate(ios_base::badbit | ios_base::failbit);
-    }
-#  if _LIBCPP_HAS_EXCEPTIONS
-  } catch (...) {
-    this->__set_badbit_and_consider_rethrow();
-  }
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-  return *this;
+  return __put_num_integer_promote(__n);
 }
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::operator<<(long __n) {
-#  if _LIBCPP_HAS_EXCEPTIONS
-  try {
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-    sentry __s(*this);
-    if (__s) {
-      typedef num_put<char_type, ostreambuf_iterator<char_type, traits_type> > _Fp;
-      const _Fp& __f = std::use_facet<_Fp>(this->getloc());
-      if (__f.put(*this, *this, this->fill(), __n).failed())
-        this->setstate(ios_base::badbit | ios_base::failbit);
-    }
-#  if _LIBCPP_HAS_EXCEPTIONS
-  } catch (...) {
-    this->__set_badbit_and_consider_rethrow();
-  }
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-  return *this;
+  return __put_num(__n);
 }
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::operator<<(unsigned long __n) {
-#  if _LIBCPP_HAS_EXCEPTIONS
-  try {
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-    sentry __s(*this);
-    if (__s) {
-      typedef num_put<char_type, ostreambuf_iterator<char_type, traits_type> > _Fp;
-      const _Fp& __f = std::use_facet<_Fp>(this->getloc());
-      if (__f.put(*this, *this, this->fill(), __n).failed())
-        this->setstate(ios_base::badbit | ios_base::failbit);
-    }
-#  if _LIBCPP_HAS_EXCEPTIONS
-  } catch (...) {
-    this->__set_badbit_and_consider_rethrow();
-  }
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-  return *this;
+  return __put_num(__n);
 }
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::operator<<(long long __n) {
-#  if _LIBCPP_HAS_EXCEPTIONS
-  try {
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-    sentry __s(*this);
-    if (__s) {
-      typedef num_put<char_type, ostreambuf_iterator<char_type, traits_type> > _Fp;
-      const _Fp& __f = std::use_facet<_Fp>(this->getloc());
-      if (__f.put(*this, *this, this->fill(), __n).failed())
-        this->setstate(ios_base::badbit | ios_base::failbit);
-    }
-#  if _LIBCPP_HAS_EXCEPTIONS
-  } catch (...) {
-    this->__set_badbit_and_consider_rethrow();
-  }
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-  return *this;
+  return __put_num(__n);
 }
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::operator<<(unsigned long long __n) {
-#  if _LIBCPP_HAS_EXCEPTIONS
-  try {
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-    sentry __s(*this);
-    if (__s) {
-      typedef num_put<char_type, ostreambuf_iterator<char_type, traits_type> > _Fp;
-      const _Fp& __f = std::use_facet<_Fp>(this->getloc());
-      if (__f.put(*this, *this, this->fill(), __n).failed())
-        this->setstate(ios_base::badbit | ios_base::failbit);
-    }
-#  if _LIBCPP_HAS_EXCEPTIONS
-  } catch (...) {
-    this->__set_badbit_and_consider_rethrow();
-  }
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-  return *this;
+  return __put_num(__n);
 }
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::operator<<(float __n) {
-#  if _LIBCPP_HAS_EXCEPTIONS
-  try {
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-    sentry __s(*this);
-    if (__s) {
-      typedef num_put<char_type, ostreambuf_iterator<char_type, traits_type> > _Fp;
-      const _Fp& __f = std::use_facet<_Fp>(this->getloc());
-      if (__f.put(*this, *this, this->fill(), static_cast<double>(__n)).failed())
-        this->setstate(ios_base::badbit | ios_base::failbit);
-    }
-#  if _LIBCPP_HAS_EXCEPTIONS
-  } catch (...) {
-    this->__set_badbit_and_consider_rethrow();
-  }
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-  return *this;
+  return *this << static_cast<double>(__n);
 }
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::operator<<(double __n) {
-#  if _LIBCPP_HAS_EXCEPTIONS
-  try {
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-    sentry __s(*this);
-    if (__s) {
-      typedef num_put<char_type, ostreambuf_iterator<char_type, traits_type> > _Fp;
-      const _Fp& __f = std::use_facet<_Fp>(this->getloc());
-      if (__f.put(*this, *this, this->fill(), __n).failed())
-        this->setstate(ios_base::badbit | ios_base::failbit);
-    }
-#  if _LIBCPP_HAS_EXCEPTIONS
-  } catch (...) {
-    this->__set_badbit_and_consider_rethrow();
-  }
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-  return *this;
+  return __put_num(__n);
 }
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::operator<<(long double __n) {
-#  if _LIBCPP_HAS_EXCEPTIONS
-  try {
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-    sentry __s(*this);
-    if (__s) {
-      typedef num_put<char_type, ostreambuf_iterator<char_type, traits_type> > _Fp;
-      const _Fp& __f = std::use_facet<_Fp>(this->getloc());
-      if (__f.put(*this, *this, this->fill(), __n).failed())
-        this->setstate(ios_base::badbit | ios_base::failbit);
-    }
-#  if _LIBCPP_HAS_EXCEPTIONS
-  } catch (...) {
-    this->__set_badbit_and_consider_rethrow();
-  }
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-  return *this;
+  return __put_num(__n);
 }
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::operator<<(const void* __n) {
-#  if _LIBCPP_HAS_EXCEPTIONS
-  try {
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-    sentry __s(*this);
-    if (__s) {
-      typedef num_put<char_type, ostreambuf_iterator<char_type, traits_type> > _Fp;
-      const _Fp& __f = std::use_facet<_Fp>(this->getloc());
-      if (__f.put(*this, *this, this->fill(), __n).failed())
-        this->setstate(ios_base::badbit | ios_base::failbit);
-    }
-#  if _LIBCPP_HAS_EXCEPTIONS
-  } catch (...) {
-    this->__set_badbit_and_consider_rethrow();
-  }
-#  endif // _LIBCPP_HAS_EXCEPTIONS
-  return *this;
+  return __put_num(__n);
 }
 
 template <class _CharT, class _Traits>
diff --git a/libcxx/include/__pstl/backend_fwd.h b/libcxx/include/__pstl/backend_fwd.h
index 2132e8dbceb3..a7d53b6a1c98 100644
--- a/libcxx/include/__pstl/backend_fwd.h
+++ b/libcxx/include/__pstl/backend_fwd.h
@@ -53,11 +53,13 @@ struct __serial_backend_tag;
 struct __std_thread_backend_tag;
 
 #  if defined(_LIBCPP_PSTL_BACKEND_SERIAL)
-using __current_configuration = __backend_configuration<__serial_backend_tag, __default_backend_tag>;
+using __current_configuration _LIBCPP_NODEBUG = __backend_configuration<__serial_backend_tag, __default_backend_tag>;
 #  elif defined(_LIBCPP_PSTL_BACKEND_STD_THREAD)
-using __current_configuration = __backend_configuration<__std_thread_backend_tag, __default_backend_tag>;
+using __current_configuration _LIBCPP_NODEBUG =
+    __backend_configuration<__std_thread_backend_tag, __default_backend_tag>;
 #  elif defined(_LIBCPP_PSTL_BACKEND_LIBDISPATCH)
-using __current_configuration = __backend_configuration<__libdispatch_backend_tag, __default_backend_tag>;
+using __current_configuration _LIBCPP_NODEBUG =
+    __backend_configuration<__libdispatch_backend_tag, __default_backend_tag>;
 #  else
 
 // ...New vendors can add parallel backends here...
diff --git a/libcxx/include/__pstl/dispatch.h b/libcxx/include/__pstl/dispatch.h
index ea40fa79eb94..828842368e33 100644
--- a/libcxx/include/__pstl/dispatch.h
+++ b/libcxx/include/__pstl/dispatch.h
@@ -58,7 +58,8 @@ struct __find_first_implemented<_Algorithm, __backend_configuration<_B1, _Bn...>
           __find_first_implemented<_Algorithm, __backend_configuration<_Bn...>, _ExecutionPolicy> > {};
 
 template <template <class, class> class _Algorithm, class _BackendConfiguration, class _ExecutionPolicy>
-using __dispatch = typename __find_first_implemented<_Algorithm, _BackendConfiguration, _ExecutionPolicy>::type;
+using __dispatch _LIBCPP_NODEBUG =
+    typename __find_first_implemented<_Algorithm, _BackendConfiguration, _ExecutionPolicy>::type;
 
 } // namespace __pstl
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/__ranges/chunk_by_view.h b/libcxx/include/__ranges/chunk_by_view.h
index e4df589bc286..71fee3a4f2d1 100644
--- a/libcxx/include/__ranges/chunk_by_view.h
+++ b/libcxx/include/__ranges/chunk_by_view.h
@@ -59,7 +59,7 @@ class _LIBCPP_ABI_LLVM18_NO_UNIQUE_ADDRESS chunk_by_view : public view_interface
   _LIBCPP_NO_UNIQUE_ADDRESS __movable_box<_Pred> __pred_;
 
   // We cache the result of begin() to allow providing an amortized O(1).
-  using _Cache = __non_propagating_cache<iterator_t<_View>>;
+  using _Cache _LIBCPP_NODEBUG = __non_propagating_cache<iterator_t<_View>>;
   _Cache __cached_begin_;
 
   class __iterator;
diff --git a/libcxx/include/__ranges/drop_view.h b/libcxx/include/__ranges/drop_view.h
index 87f66f17a2ab..3f963d04fff2 100644
--- a/libcxx/include/__ranges/drop_view.h
+++ b/libcxx/include/__ranges/drop_view.h
@@ -64,7 +64,7 @@ class drop_view : public view_interface<drop_view<_View>> {
   // Note: drop_view<input-range>::begin() is still trivially amortized O(1) because
   // one can't call begin() on it more than once.
   static constexpr bool _UseCache = forward_range<_View> && !(random_access_range<_View> && sized_range<_View>);
-  using _Cache                    = _If<_UseCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
+  using _Cache _LIBCPP_NODEBUG    = _If<_UseCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
   _LIBCPP_NO_UNIQUE_ADDRESS _Cache __cached_begin_ = _Cache();
   range_difference_t<_View> __count_               = 0;
   _View __base_                                    = _View();
@@ -204,7 +204,7 @@ struct __passthrough_type<subrange<_Iter, _Sent, _Kind>> {
 };
 
 template <class _Tp>
-using __passthrough_type_t = typename __passthrough_type<_Tp>::type;
+using __passthrough_type_t _LIBCPP_NODEBUG = typename __passthrough_type<_Tp>::type;
 
 struct __fn {
   // [range.drop.overview]: the `empty_view` case.
diff --git a/libcxx/include/__ranges/drop_while_view.h b/libcxx/include/__ranges/drop_while_view.h
index 6413ff52bc0e..bc7f019393a8 100644
--- a/libcxx/include/__ranges/drop_while_view.h
+++ b/libcxx/include/__ranges/drop_while_view.h
@@ -90,7 +90,7 @@ private:
   _LIBCPP_NO_UNIQUE_ADDRESS __movable_box<_Pred> __pred_;
 
   static constexpr bool _UseCache = forward_range<_View>;
-  using _Cache                    = _If<_UseCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
+  using _Cache _LIBCPP_NODEBUG    = _If<_UseCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
   _LIBCPP_NO_UNIQUE_ADDRESS _Cache __cached_begin_ = _Cache();
 };
 
diff --git a/libcxx/include/__ranges/elements_view.h b/libcxx/include/__ranges/elements_view.h
index c99282f37960..5121298fb684 100644
--- a/libcxx/include/__ranges/elements_view.h
+++ b/libcxx/include/__ranges/elements_view.h
@@ -171,7 +171,7 @@ class elements_view<_View, _Np>::__iterator
   template <bool>
   friend class __sentinel;
 
-  using _Base = __maybe_const<_Const, _View>;
+  using _Base _LIBCPP_NODEBUG = __maybe_const<_Const, _View>;
 
   iterator_t<_Base> __current_ = iterator_t<_Base>();
 
@@ -335,7 +335,7 @@ template <input_range _View, size_t _Np>
 template <bool _Const>
 class elements_view<_View, _Np>::__sentinel {
 private:
-  using _Base                                        = __maybe_const<_Const, _View>;
+  using _Base _LIBCPP_NODEBUG                        = __maybe_const<_Const, _View>;
   _LIBCPP_NO_UNIQUE_ADDRESS sentinel_t<_Base> __end_ = sentinel_t<_Base>();
 
   template <bool>
diff --git a/libcxx/include/__ranges/filter_view.h b/libcxx/include/__ranges/filter_view.h
index 22f67b2d967d..07980e735319 100644
--- a/libcxx/include/__ranges/filter_view.h
+++ b/libcxx/include/__ranges/filter_view.h
@@ -61,7 +61,7 @@ class _LIBCPP_ABI_LLVM18_NO_UNIQUE_ADDRESS filter_view : public view_interface<f
   // We cache the result of begin() to allow providing an amortized O(1) begin() whenever
   // the underlying range is at least a forward_range.
   static constexpr bool _UseCache = forward_range<_View>;
-  using _Cache                    = _If<_UseCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
+  using _Cache _LIBCPP_NODEBUG    = _If<_UseCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
   _LIBCPP_NO_UNIQUE_ADDRESS _Cache __cached_begin_ = _Cache();
 
   class __iterator;
@@ -115,7 +115,7 @@ struct __filter_iterator_category {};
 
 template <forward_range _View>
 struct __filter_iterator_category<_View> {
-  using _Cat = typename iterator_traits<iterator_t<_View>>::iterator_category;
+  using _Cat _LIBCPP_NODEBUG = typename iterator_traits<iterator_t<_View>>::iterator_category;
   using iterator_category =
       _If<derived_from<_Cat, bidirectional_iterator_tag>,
           bidirectional_iterator_tag,
diff --git a/libcxx/include/__ranges/iota_view.h b/libcxx/include/__ranges/iota_view.h
index b2fa958a0f56..4b84585258b9 100644
--- a/libcxx/include/__ranges/iota_view.h
+++ b/libcxx/include/__ranges/iota_view.h
@@ -68,7 +68,7 @@ struct __get_wider_signed {
 };
 
 template <class _Start>
-using _IotaDiffT =
+using _IotaDiffT _LIBCPP_NODEBUG =
     typename _If< (!integral<_Start> || sizeof(iter_difference_t<_Start>) > sizeof(_Start)),
                   type_identity<iter_difference_t<_Start>>,
                   __get_wider_signed<_Start> >::type;
diff --git a/libcxx/include/__ranges/join_view.h b/libcxx/include/__ranges/join_view.h
index 6aadd387860e..327b349f476a 100644
--- a/libcxx/include/__ranges/join_view.h
+++ b/libcxx/include/__ranges/join_view.h
@@ -55,8 +55,8 @@ struct __join_view_iterator_category {};
 template <class _View>
   requires is_reference_v<range_reference_t<_View>> && forward_range<_View> && forward_range<range_reference_t<_View>>
 struct __join_view_iterator_category<_View> {
-  using _OuterC = typename iterator_traits<iterator_t<_View>>::iterator_category;
-  using _InnerC = typename iterator_traits<iterator_t<range_reference_t<_View>>>::iterator_category;
+  using _OuterC _LIBCPP_NODEBUG = typename iterator_traits<iterator_t<_View>>::iterator_category;
+  using _InnerC _LIBCPP_NODEBUG = typename iterator_traits<iterator_t<range_reference_t<_View>>>::iterator_category;
 
   using iterator_category =
       _If< derived_from<_OuterC, bidirectional_iterator_tag> && derived_from<_InnerC, bidirectional_iterator_tag> &&
@@ -71,7 +71,7 @@ template <input_range _View>
   requires view<_View> && input_range<range_reference_t<_View>>
 class join_view : public view_interface<join_view<_View>> {
 private:
-  using _InnerRange = range_reference_t<_View>;
+  using _InnerRange _LIBCPP_NODEBUG = range_reference_t<_View>;
 
   template <bool>
   struct __iterator;
@@ -85,11 +85,12 @@ private:
   _LIBCPP_NO_UNIQUE_ADDRESS _View __base_ = _View();
 
   static constexpr bool _UseOuterCache = !forward_range<_View>;
-  using _OuterCache                    = _If<_UseOuterCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
+  using _OuterCache _LIBCPP_NODEBUG    = _If<_UseOuterCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
   _LIBCPP_NO_UNIQUE_ADDRESS _OuterCache __outer_;
 
   static constexpr bool _UseInnerCache = !is_reference_v<_InnerRange>;
-  using _InnerCache = _If<_UseInnerCache, __non_propagating_cache<remove_cvref_t<_InnerRange>>, __empty_cache>;
+  using _InnerCache _LIBCPP_NODEBUG =
+      _If<_UseInnerCache, __non_propagating_cache<remove_cvref_t<_InnerRange>>, __empty_cache>;
   _LIBCPP_NO_UNIQUE_ADDRESS _InnerCache __inner_;
 
 public:
@@ -155,9 +156,9 @@ private:
   template <bool>
   friend struct __sentinel;
 
-  using _Parent            = __maybe_const<_Const, join_view>;
-  using _Base              = __maybe_const<_Const, _View>;
-  sentinel_t<_Base> __end_ = sentinel_t<_Base>();
+  using _Parent _LIBCPP_NODEBUG = __maybe_const<_Const, join_view>;
+  using _Base _LIBCPP_NODEBUG   = __maybe_const<_Const, _View>;
+  sentinel_t<_Base> __end_      = sentinel_t<_Base>();
 
 public:
   _LIBCPP_HIDE_FROM_ABI __sentinel() = default;
@@ -190,18 +191,18 @@ struct join_view<_View>::__iterator final : public __join_view_iterator_category
   static constexpr bool __is_join_view_iterator = true;
 
 private:
-  using _Parent     = __maybe_const<_Const, join_view<_View>>;
-  using _Base       = __maybe_const<_Const, _View>;
-  using _Outer      = iterator_t<_Base>;
-  using _Inner      = iterator_t<range_reference_t<_Base>>;
-  using _InnerRange = range_reference_t<_View>;
+  using _Parent _LIBCPP_NODEBUG     = __maybe_const<_Const, join_view<_View>>;
+  using _Base _LIBCPP_NODEBUG       = __maybe_const<_Const, _View>;
+  using _Outer _LIBCPP_NODEBUG      = iterator_t<_Base>;
+  using _Inner _LIBCPP_NODEBUG      = iterator_t<range_reference_t<_Base>>;
+  using _InnerRange _LIBCPP_NODEBUG = range_reference_t<_View>;
 
   static_assert(!_Const || forward_range<_Base>, "Const can only be true when Base models forward_range.");
 
   static constexpr bool __ref_is_glvalue = is_reference_v<range_reference_t<_Base>>;
 
   static constexpr bool _OuterPresent           = forward_range<_Base>;
-  using _OuterType                              = _If<_OuterPresent, _Outer, std::__empty>;
+  using _OuterType _LIBCPP_NODEBUG              = _If<_OuterPresent, _Outer, std::__empty>;
   _LIBCPP_NO_UNIQUE_ADDRESS _OuterType __outer_ = _OuterType();
 
   optional<_Inner> __inner_;
@@ -379,7 +380,7 @@ template <class _JoinViewIterator>
 struct __segmented_iterator_traits<_JoinViewIterator> {
   using __segment_iterator _LIBCPP_NODEBUG =
       __iterator_with_data<typename _JoinViewIterator::_Outer, typename _JoinViewIterator::_Parent*>;
-  using __local_iterator = typename _JoinViewIterator::_Inner;
+  using __local_iterator _LIBCPP_NODEBUG = typename _JoinViewIterator::_Inner;
 
   // TODO: Would it make sense to enable the optimization for other iterator types?
 
diff --git a/libcxx/include/__ranges/lazy_split_view.h b/libcxx/include/__ranges/lazy_split_view.h
index 0dcbc134a21f..cca9191d2681 100644
--- a/libcxx/include/__ranges/lazy_split_view.h
+++ b/libcxx/include/__ranges/lazy_split_view.h
@@ -72,7 +72,8 @@ class lazy_split_view : public view_interface<lazy_split_view<_View, _Pattern>>
   _LIBCPP_NO_UNIQUE_ADDRESS _View __base_       = _View();
   _LIBCPP_NO_UNIQUE_ADDRESS _Pattern __pattern_ = _Pattern();
 
-  using _MaybeCurrent = _If<!forward_range<_View>, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
+  using _MaybeCurrent _LIBCPP_NODEBUG =
+      _If<!forward_range<_View>, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
   _LIBCPP_NO_UNIQUE_ADDRESS _MaybeCurrent __current_ = _MaybeCurrent();
 
   template <bool>
@@ -146,11 +147,11 @@ private:
     friend struct __inner_iterator;
     friend __outer_iterator<true>;
 
-    using _Parent = __maybe_const<_Const, lazy_split_view>;
-    using _Base   = __maybe_const<_Const, _View>;
+    using _Parent _LIBCPP_NODEBUG = __maybe_const<_Const, lazy_split_view>;
+    using _Base _LIBCPP_NODEBUG   = __maybe_const<_Const, _View>;
 
     _Parent* __parent_                                 = nullptr;
-    using _MaybeCurrent                                = _If<forward_range<_View>, iterator_t<_Base>, __empty_cache>;
+    using _MaybeCurrent _LIBCPP_NODEBUG                = _If<forward_range<_View>, iterator_t<_Base>, __empty_cache>;
     _LIBCPP_NO_UNIQUE_ADDRESS _MaybeCurrent __current_ = _MaybeCurrent();
     bool __trailing_empty_                             = false;
 
@@ -283,7 +284,7 @@ private:
   template <bool _Const>
   struct __inner_iterator : __inner_iterator_category<__maybe_const<_Const, _View>> {
   private:
-    using _Base = __maybe_const<_Const, _View>;
+    using _Base _LIBCPP_NODEBUG = __maybe_const<_Const, _View>;
     // Workaround for a GCC issue.
     static constexpr bool _OuterConst = _Const;
     __outer_iterator<_Const> __i_     = __outer_iterator<_OuterConst>();
diff --git a/libcxx/include/__ranges/repeat_view.h b/libcxx/include/__ranges/repeat_view.h
index 93ceaf1711d3..61a8b6357105 100644
--- a/libcxx/include/__ranges/repeat_view.h
+++ b/libcxx/include/__ranges/repeat_view.h
@@ -61,7 +61,7 @@ struct __repeat_view_iterator_difference<_Tp> {
 };
 
 template <class _Tp>
-using __repeat_view_iterator_difference_t = typename __repeat_view_iterator_difference<_Tp>::type;
+using __repeat_view_iterator_difference_t _LIBCPP_NODEBUG = typename __repeat_view_iterator_difference<_Tp>::type;
 
 namespace views::__drop {
 struct __fn;
@@ -139,7 +139,7 @@ template <move_constructible _Tp, semiregular _Bound>
 class repeat_view<_Tp, _Bound>::__iterator {
   friend class repeat_view;
 
-  using _IndexT = conditional_t<same_as<_Bound, unreachable_sentinel_t>, ptrdiff_t, _Bound>;
+  using _IndexT _LIBCPP_NODEBUG = conditional_t<same_as<_Bound, unreachable_sentinel_t>, ptrdiff_t, _Bound>;
 
   _LIBCPP_HIDE_FROM_ABI constexpr explicit __iterator(const _Tp* __value, _IndexT __bound_sentinel = _IndexT())
       : __value_(__value), __current_(__bound_sentinel) {}
diff --git a/libcxx/include/__ranges/reverse_view.h b/libcxx/include/__ranges/reverse_view.h
index 796f5be22328..80d54b9a6c83 100644
--- a/libcxx/include/__ranges/reverse_view.h
+++ b/libcxx/include/__ranges/reverse_view.h
@@ -47,7 +47,8 @@ class reverse_view : public view_interface<reverse_view<_View>> {
   // We cache begin() whenever ranges::next is not guaranteed O(1) to provide an
   // amortized O(1) begin() method.
   static constexpr bool _UseCache = !random_access_range<_View> && !common_range<_View>;
-  using _Cache = _If<_UseCache, __non_propagating_cache<reverse_iterator<iterator_t<_View>>>, __empty_cache>;
+  using _Cache _LIBCPP_NODEBUG =
+      _If<_UseCache, __non_propagating_cache<reverse_iterator<iterator_t<_View>>>, __empty_cache>;
   _LIBCPP_NO_UNIQUE_ADDRESS _Cache __cached_begin_ = _Cache();
   _LIBCPP_NO_UNIQUE_ADDRESS _View __base_          = _View();
 
diff --git a/libcxx/include/__ranges/split_view.h b/libcxx/include/__ranges/split_view.h
index 7527281905ff..2ec908ba4070 100644
--- a/libcxx/include/__ranges/split_view.h
+++ b/libcxx/include/__ranges/split_view.h
@@ -52,7 +52,7 @@ class split_view : public view_interface<split_view<_View, _Pattern>> {
 private:
   _LIBCPP_NO_UNIQUE_ADDRESS _View __base_       = _View();
   _LIBCPP_NO_UNIQUE_ADDRESS _Pattern __pattern_ = _Pattern();
-  using _Cache                                  = __non_propagating_cache<subrange<iterator_t<_View>>>;
+  using _Cache _LIBCPP_NODEBUG                  = __non_propagating_cache<subrange<iterator_t<_View>>>;
   _Cache __cached_begin_                        = _Cache();
 
   template <class, class>
diff --git a/libcxx/include/__ranges/subrange.h b/libcxx/include/__ranges/subrange.h
index a40eab3c5a25..2d006d3570a7 100644
--- a/libcxx/include/__ranges/subrange.h
+++ b/libcxx/include/__ranges/subrange.h
@@ -82,7 +82,7 @@ private:
   struct _Empty {
     _LIBCPP_HIDE_FROM_ABI constexpr _Empty(auto) noexcept {}
   };
-  using _Size = conditional_t<_StoreSize, make_unsigned_t<iter_difference_t<_Iter>>, _Empty>;
+  using _Size _LIBCPP_NODEBUG = conditional_t<_StoreSize, make_unsigned_t<iter_difference_t<_Iter>>, _Empty>;
   _LIBCPP_NO_UNIQUE_ADDRESS _Iter __begin_ = _Iter();
   _LIBCPP_NO_UNIQUE_ADDRESS _Sent __end_   = _Sent();
   _LIBCPP_NO_UNIQUE_ADDRESS _Size __size_  = 0;
diff --git a/libcxx/include/__ranges/take_view.h b/libcxx/include/__ranges/take_view.h
index 39f99cee6b4d..5892c1e31fae 100644
--- a/libcxx/include/__ranges/take_view.h
+++ b/libcxx/include/__ranges/take_view.h
@@ -161,9 +161,9 @@ public:
 template <view _View>
 template <bool _Const>
 class take_view<_View>::__sentinel {
-  using _Base = __maybe_const<_Const, _View>;
+  using _Base _LIBCPP_NODEBUG = __maybe_const<_Const, _View>;
   template <bool _OtherConst>
-  using _Iter                                        = counted_iterator<iterator_t<__maybe_const<_OtherConst, _View>>>;
+  using _Iter _LIBCPP_NODEBUG                        = counted_iterator<iterator_t<__maybe_const<_OtherConst, _View>>>;
   _LIBCPP_NO_UNIQUE_ADDRESS sentinel_t<_Base> __end_ = sentinel_t<_Base>();
 
   template <bool>
@@ -244,7 +244,7 @@ struct __passthrough_type<subrange<_Iter, _Sent, _Kind>> {
 };
 
 template <class _Tp>
-using __passthrough_type_t = typename __passthrough_type<_Tp>::type;
+using __passthrough_type_t _LIBCPP_NODEBUG = typename __passthrough_type<_Tp>::type;
 
 struct __fn {
   // [range.take.overview]: the `empty_view` case.
diff --git a/libcxx/include/__ranges/take_while_view.h b/libcxx/include/__ranges/take_while_view.h
index b7cb0aef58f1..4977f139fc55 100644
--- a/libcxx/include/__ranges/take_while_view.h
+++ b/libcxx/include/__ranges/take_while_view.h
@@ -103,7 +103,7 @@ template <view _View, class _Pred>
   requires input_range<_View> && is_object_v<_Pred> && indirect_unary_predicate<const _Pred, iterator_t<_View>>
 template <bool _Const>
 class take_while_view<_View, _Pred>::__sentinel {
-  using _Base = __maybe_const<_Const, _View>;
+  using _Base _LIBCPP_NODEBUG = __maybe_const<_Const, _View>;
 
   sentinel_t<_Base> __end_ = sentinel_t<_Base>();
   const _Pred* __pred_     = nullptr;
diff --git a/libcxx/include/__ranges/transform_view.h b/libcxx/include/__ranges/transform_view.h
index 0ce1bfe88350..4ae21e92b69d 100644
--- a/libcxx/include/__ranges/transform_view.h
+++ b/libcxx/include/__ranges/transform_view.h
@@ -159,7 +159,7 @@ struct __transform_view_iterator_category_base {};
 
 template <forward_range _View, class _Fn>
 struct __transform_view_iterator_category_base<_View, _Fn> {
-  using _Cat = typename iterator_traits<iterator_t<_View>>::iterator_category;
+  using _Cat _LIBCPP_NODEBUG = typename iterator_traits<iterator_t<_View>>::iterator_category;
 
   using iterator_category =
       conditional_t< is_reference_v<invoke_result_t<_Fn&, range_reference_t<_View>>>,
@@ -177,8 +177,8 @@ template <bool _Const>
 class transform_view<_View, _Fn>::__iterator
     : public __transform_view_iterator_category_base<_View, __maybe_const<_Const, _Fn>> {
 
-  using _Parent = __maybe_const<_Const, transform_view>;
-  using _Base   = __maybe_const<_Const, _View>;
+  using _Parent _LIBCPP_NODEBUG = __maybe_const<_Const, transform_view>;
+  using _Base _LIBCPP_NODEBUG   = __maybe_const<_Const, _View>;
 
   _Parent* __parent_ = nullptr;
 
@@ -338,8 +338,8 @@ template <input_range _View, copy_constructible _Fn>
   requires __transform_view_constraints<_View, _Fn>
 template <bool _Const>
 class transform_view<_View, _Fn>::__sentinel {
-  using _Parent = __maybe_const<_Const, transform_view>;
-  using _Base   = __maybe_const<_Const, _View>;
+  using _Parent _LIBCPP_NODEBUG = __maybe_const<_Const, transform_view>;
+  using _Base _LIBCPP_NODEBUG   = __maybe_const<_Const, _View>;
 
   sentinel_t<_Base> __end_ = sentinel_t<_Base>();
 
diff --git a/libcxx/include/__split_buffer b/libcxx/include/__split_buffer
index 9d7fd3e0df62..a8f679cc30a9 100644
--- a/libcxx/include/__split_buffer
+++ b/libcxx/include/__split_buffer
@@ -51,24 +51,24 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Tp, class _Allocator = allocator<_Tp> >
 struct __split_buffer {
 public:
-  using value_type      = _Tp;
-  using allocator_type  = _Allocator;
-  using __alloc_rr      = __libcpp_remove_reference_t<allocator_type>;
-  using __alloc_traits  = allocator_traits<__alloc_rr>;
-  using reference       = value_type&;
-  using const_reference = const value_type&;
-  using size_type       = typename __alloc_traits::size_type;
-  using difference_type = typename __alloc_traits::difference_type;
-  using pointer         = typename __alloc_traits::pointer;
-  using const_pointer   = typename __alloc_traits::const_pointer;
-  using iterator        = pointer;
-  using const_iterator  = const_pointer;
+  using value_type                     = _Tp;
+  using allocator_type                 = _Allocator;
+  using __alloc_rr _LIBCPP_NODEBUG     = __libcpp_remove_reference_t<allocator_type>;
+  using __alloc_traits _LIBCPP_NODEBUG = allocator_traits<__alloc_rr>;
+  using reference                      = value_type&;
+  using const_reference                = const value_type&;
+  using size_type                      = typename __alloc_traits::size_type;
+  using difference_type                = typename __alloc_traits::difference_type;
+  using pointer                        = typename __alloc_traits::pointer;
+  using const_pointer                  = typename __alloc_traits::const_pointer;
+  using iterator                       = pointer;
+  using const_iterator                 = const_pointer;
 
   // A __split_buffer contains the following members which may be trivially relocatable:
   // - pointer: may be trivially relocatable, so it's checked
   // - allocator_type: may be trivially relocatable, so it's checked
   // __split_buffer doesn't have any self-references, so it's trivially relocatable if its members are.
-  using __trivially_relocatable = __conditional_t<
+  using __trivially_relocatable _LIBCPP_NODEBUG = __conditional_t<
       __libcpp_is_trivially_relocatable<pointer>::value && __libcpp_is_trivially_relocatable<allocator_type>::value,
       __split_buffer,
       void>;
diff --git a/libcxx/include/__stop_token/stop_state.h b/libcxx/include/__stop_token/stop_state.h
index 84dc208dda36..cc1f1d830e8d 100644
--- a/libcxx/include/__stop_token/stop_state.h
+++ b/libcxx/include/__stop_token/stop_state.h
@@ -27,7 +27,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #if _LIBCPP_STD_VER >= 20 && _LIBCPP_HAS_THREADS
 
 struct __stop_callback_base : __intrusive_node_base<__stop_callback_base> {
-  using __callback_fn_t = void(__stop_callback_base*) noexcept;
+  using __callback_fn_t _LIBCPP_NODEBUG = void(__stop_callback_base*) noexcept;
   _LIBCPP_HIDE_FROM_ABI explicit __stop_callback_base(__callback_fn_t* __callback_fn) : __callback_fn_(__callback_fn) {}
 
   _LIBCPP_HIDE_FROM_ABI void __invoke() noexcept { __callback_fn_(this); }
@@ -58,9 +58,9 @@ class __stop_state {
   // It is used by __intrusive_shared_ptr, but it is stored here for better layout
   atomic<uint32_t> __ref_count_ = 0;
 
-  using __state_t            = uint32_t;
-  using __callback_list_lock = __atomic_unique_lock<__state_t, __callback_list_locked_bit>;
-  using __callback_list      = __intrusive_list_view<__stop_callback_base>;
+  using __state_t _LIBCPP_NODEBUG            = uint32_t;
+  using __callback_list_lock _LIBCPP_NODEBUG = __atomic_unique_lock<__state_t, __callback_list_locked_bit>;
+  using __callback_list _LIBCPP_NODEBUG      = __intrusive_list_view<__stop_callback_base>;
 
   __callback_list __callback_list_;
   __thread_id __requesting_thread_;
diff --git a/libcxx/include/__system_error/system_error.h b/libcxx/include/__system_error/system_error.h
index 918effb6917c..36ccf94cc010 100644
--- a/libcxx/include/__system_error/system_error.h
+++ b/libcxx/include/__system_error/system_error.h
@@ -39,6 +39,10 @@ public:
   _LIBCPP_HIDE_FROM_ABI const error_code& code() const _NOEXCEPT { return __ec_; }
 };
 
+// __ev is expected to be an error in the generic_category domain (e.g. from
+// errno, or std::errc::*), not system_category (e.g. from windows syscalls).
+[[__noreturn__]] _LIBCPP_EXPORTED_FROM_ABI void __throw_system_error(int __ev, const char* __what_arg);
+
 [[__noreturn__]] _LIBCPP_HIDE_FROM_ABI inline void __throw_system_error(error_code __ec, const char* __what_arg) {
 #if _LIBCPP_HAS_EXCEPTIONS
   throw system_error(__ec, __what_arg);
diff --git a/libcxx/include/__thread/support/pthread.h b/libcxx/include/__thread/support/pthread.h
index 531f3e71de83..14e92079dadf 100644
--- a/libcxx/include/__thread/support/pthread.h
+++ b/libcxx/include/__thread/support/pthread.h
@@ -39,7 +39,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using __libcpp_timespec_t = ::timespec;
+using __libcpp_timespec_t _LIBCPP_NODEBUG = ::timespec;
 
 //
 // Mutex
diff --git a/libcxx/include/__tuple/make_tuple_types.h b/libcxx/include/__tuple/make_tuple_types.h
index 3d312395131d..ff95ca4313a5 100644
--- a/libcxx/include/__tuple/make_tuple_types.h
+++ b/libcxx/include/__tuple/make_tuple_types.h
@@ -47,9 +47,9 @@ struct __make_tuple_types_flat<_Tuple<_Types...>, __tuple_indices<_Idx...>> {
 template <class _Vt, size_t _Np, size_t... _Idx>
 struct __make_tuple_types_flat<array<_Vt, _Np>, __tuple_indices<_Idx...>> {
   template <size_t>
-  using __value_type = _Vt;
+  using __value_type _LIBCPP_NODEBUG = _Vt;
   template <class _Tp>
-  using __apply_quals = __tuple_types<__copy_cvref_t<_Tp, __value_type<_Idx>>...>;
+  using __apply_quals _LIBCPP_NODEBUG = __tuple_types<__copy_cvref_t<_Tp, __value_type<_Idx>>...>;
 };
 
 template <class _Tp,
@@ -58,9 +58,9 @@ template <class _Tp,
           bool _SameSize = (_Ep == tuple_size<__libcpp_remove_reference_t<_Tp> >::value)>
 struct __make_tuple_types {
   static_assert(_Sp <= _Ep, "__make_tuple_types input error");
-  using _RawTp = __remove_cvref_t<_Tp>;
-  using _Maker = __make_tuple_types_flat<_RawTp, typename __make_tuple_indices<_Ep, _Sp>::type>;
-  using type   = typename _Maker::template __apply_quals<_Tp>;
+  using _RawTp _LIBCPP_NODEBUG = __remove_cvref_t<_Tp>;
+  using _Maker _LIBCPP_NODEBUG = __make_tuple_types_flat<_RawTp, typename __make_tuple_indices<_Ep, _Sp>::type>;
+  using type                   = typename _Maker::template __apply_quals<_Tp>;
 };
 
 template <class... _Types, size_t _Ep>
diff --git a/libcxx/include/__tuple/sfinae_helpers.h b/libcxx/include/__tuple/sfinae_helpers.h
index 9041d1d4473e..4084e8bb31fd 100644
--- a/libcxx/include/__tuple/sfinae_helpers.h
+++ b/libcxx/include/__tuple/sfinae_helpers.h
@@ -41,7 +41,7 @@ struct __tuple_sfinae_base {
   static auto __do_test(...) -> false_type;
 
   template <class _FromArgs, class _ToArgs>
-  using __constructible = decltype(__do_test<is_constructible>(_ToArgs{}, _FromArgs{}));
+  using __constructible _LIBCPP_NODEBUG = decltype(__do_test<is_constructible>(_ToArgs{}, _FromArgs{}));
 };
 
 // __tuple_constructible
diff --git a/libcxx/include/__tuple/tuple_size.h b/libcxx/include/__tuple/tuple_size.h
index b970280fe378..27d57eb56ba6 100644
--- a/libcxx/include/__tuple/tuple_size.h
+++ b/libcxx/include/__tuple/tuple_size.h
@@ -29,7 +29,7 @@ struct _LIBCPP_TEMPLATE_VIS tuple_size;
 
 #if !defined(_LIBCPP_CXX03_LANG)
 template <class _Tp, class...>
-using __enable_if_tuple_size_imp = _Tp;
+using __enable_if_tuple_size_imp _LIBCPP_NODEBUG = _Tp;
 
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp< const _Tp,
diff --git a/libcxx/include/__type_traits/add_lvalue_reference.h b/libcxx/include/__type_traits/add_lvalue_reference.h
index 157c8f94d476..b1ee6ed73c8a 100644
--- a/libcxx/include/__type_traits/add_lvalue_reference.h
+++ b/libcxx/include/__type_traits/add_lvalue_reference.h
@@ -21,7 +21,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #if __has_builtin(__add_lvalue_reference)
 
 template <class _Tp>
-using __add_lvalue_reference_t = __add_lvalue_reference(_Tp);
+using __add_lvalue_reference_t _LIBCPP_NODEBUG = __add_lvalue_reference(_Tp);
 
 #else
 
diff --git a/libcxx/include/__type_traits/add_pointer.h b/libcxx/include/__type_traits/add_pointer.h
index f66e1f9e6c06..b53d8eae708e 100644
--- a/libcxx/include/__type_traits/add_pointer.h
+++ b/libcxx/include/__type_traits/add_pointer.h
@@ -23,7 +23,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #if !defined(_LIBCPP_WORKAROUND_OBJCXX_COMPILER_INTRINSICS) && __has_builtin(__add_pointer)
 
 template <class _Tp>
-using __add_pointer_t = __add_pointer(_Tp);
+using __add_pointer_t _LIBCPP_NODEBUG = __add_pointer(_Tp);
 
 #else
 template <class _Tp, bool = __libcpp_is_referenceable<_Tp>::value || is_void<_Tp>::value>
diff --git a/libcxx/include/__type_traits/add_rvalue_reference.h b/libcxx/include/__type_traits/add_rvalue_reference.h
index 205058892c87..d844ccc1f539 100644
--- a/libcxx/include/__type_traits/add_rvalue_reference.h
+++ b/libcxx/include/__type_traits/add_rvalue_reference.h
@@ -21,7 +21,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #if __has_builtin(__add_rvalue_reference)
 
 template <class _Tp>
-using __add_rvalue_reference_t = __add_rvalue_reference(_Tp);
+using __add_rvalue_reference_t _LIBCPP_NODEBUG = __add_rvalue_reference(_Tp);
 
 #else
 
diff --git a/libcxx/include/__type_traits/aligned_storage.h b/libcxx/include/__type_traits/aligned_storage.h
index 2e39afb7f880..d98749980122 100644
--- a/libcxx/include/__type_traits/aligned_storage.h
+++ b/libcxx/include/__type_traits/aligned_storage.h
@@ -34,26 +34,23 @@ struct __struct_double4 {
   double __lx[4];
 };
 
-// clang-format off
-typedef __type_list<__align_type<unsigned char>,
-        __type_list<__align_type<unsigned short>,
-        __type_list<__align_type<unsigned int>,
-        __type_list<__align_type<unsigned long>,
-        __type_list<__align_type<unsigned long long>,
-        __type_list<__align_type<double>,
-        __type_list<__align_type<long double>,
-        __type_list<__align_type<__struct_double>,
-        __type_list<__align_type<__struct_double4>,
-        __type_list<__align_type<int*>,
-        __nat
-        > > > > > > > > > > __all_types;
-// clang-format on
+using __all_types _LIBCPP_NODEBUG =
+    __type_list<__align_type<unsigned char>,
+                __align_type<unsigned short>,
+                __align_type<unsigned int>,
+                __align_type<unsigned long>,
+                __align_type<unsigned long long>,
+                __align_type<double>,
+                __align_type<long double>,
+                __align_type<__struct_double>,
+                __align_type<__struct_double4>,
+                __align_type<int*> >;
 
 template <class _TL, size_t _Len>
 struct __find_max_align;
 
-template <class _Hp, size_t _Len>
-struct __find_max_align<__type_list<_Hp, __nat>, _Len> : public integral_constant<size_t, _Hp::value> {};
+template <class _Head, size_t _Len>
+struct __find_max_align<__type_list<_Head>, _Len> : public integral_constant<size_t, _Head::value> {};
 
 template <size_t _Len, size_t _A1, size_t _A2>
 struct __select_align {
@@ -65,9 +62,11 @@ public:
   static const size_t value = _Len < __max ? __min : __max;
 };
 
-template <class _Hp, class _Tp, size_t _Len>
-struct __find_max_align<__type_list<_Hp, _Tp>, _Len>
-    : public integral_constant<size_t, __select_align<_Len, _Hp::value, __find_max_align<_Tp, _Len>::value>::value> {};
+template <class _Head, class... _Tail, size_t _Len>
+struct __find_max_align<__type_list<_Head, _Tail...>, _Len>
+    : public integral_constant<
+          size_t,
+          __select_align<_Len, _Head::value, __find_max_align<__type_list<_Tail...>, _Len>::value>::value> {};
 
 template <size_t _Len, size_t _Align = __find_max_align<__all_types, _Len>::value>
 struct _LIBCPP_DEPRECATED_IN_CXX23 _LIBCPP_TEMPLATE_VIS aligned_storage {
diff --git a/libcxx/include/__type_traits/common_reference.h b/libcxx/include/__type_traits/common_reference.h
index c802902eb19f..d436949e692f 100644
--- a/libcxx/include/__type_traits/common_reference.h
+++ b/libcxx/include/__type_traits/common_reference.h
@@ -30,7 +30,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #if _LIBCPP_STD_VER >= 20
 // Let COND_RES(X, Y) be:
 template <class _Xp, class _Yp>
-using __cond_res = decltype(false ? std::declval<_Xp (&)()>()() : std::declval<_Yp (&)()>()());
+using __cond_res _LIBCPP_NODEBUG = decltype(false ? std::declval<_Xp (&)()>()() : std::declval<_Yp (&)()>()());
 
 // Let `XREF(A)` denote a unary alias template `T` such that `T<U>` denotes the same type as `U`
 // with the addition of `A`'s cv and reference qualifiers, for a non-reference cv-unqualified type
@@ -39,7 +39,7 @@ using __cond_res = decltype(false ? std::declval<_Xp (&)()>()() : std::declval<_
 template <class _Tp>
 struct __xref {
   template <class _Up>
-  using __apply = __copy_cvref_t<_Tp, _Up>;
+  using __apply _LIBCPP_NODEBUG = __copy_cvref_t<_Tp, _Up>;
 };
 
 // Given types A and B, let X be remove_reference_t<A>, let Y be remove_reference_t<B>,
@@ -48,10 +48,10 @@ template <class _Ap, class _Bp, class _Xp = remove_reference_t<_Ap>, class _Yp =
 struct __common_ref;
 
 template <class _Xp, class _Yp>
-using __common_ref_t = typename __common_ref<_Xp, _Yp>::__type;
+using __common_ref_t _LIBCPP_NODEBUG = typename __common_ref<_Xp, _Yp>::__type;
 
 template <class _Xp, class _Yp>
-using __cv_cond_res = __cond_res<__copy_cv_t<_Xp, _Yp>&, __copy_cv_t<_Yp, _Xp>&>;
+using __cv_cond_res _LIBCPP_NODEBUG = __cond_res<__copy_cv_t<_Xp, _Yp>&, __copy_cv_t<_Yp, _Xp>&>;
 
 //    If A and B are both lvalue reference types, COMMON-REF(A, B) is
 //    COND-RES(COPYCV(X, Y)&, COPYCV(Y, X)&) if that type exists and is a reference type.
@@ -61,13 +61,13 @@ template <class _Ap, class _Bp, class _Xp, class _Yp>
     requires { typename __cv_cond_res<_Xp, _Yp>; } &&
     is_reference_v<__cv_cond_res<_Xp, _Yp>>
 struct __common_ref<_Ap&, _Bp&, _Xp, _Yp> {
-  using __type = __cv_cond_res<_Xp, _Yp>;
+  using __type _LIBCPP_NODEBUG = __cv_cond_res<_Xp, _Yp>;
 };
 // clang-format on
 
 //    Otherwise, let C be remove_reference_t<COMMON-REF(X&, Y&)>&&. ...
 template <class _Xp, class _Yp>
-using __common_ref_C = remove_reference_t<__common_ref_t<_Xp&, _Yp&>>&&;
+using __common_ref_C _LIBCPP_NODEBUG = remove_reference_t<__common_ref_t<_Xp&, _Yp&>>&&;
 
 //    .... If A and B are both rvalue reference types, C is well-formed, and
 //    is_convertible_v<A, C> && is_convertible_v<B, C> is true, then COMMON-REF(A, B) is C.
@@ -78,13 +78,13 @@ template <class _Ap, class _Bp, class _Xp, class _Yp>
     is_convertible_v<_Ap&&, __common_ref_C<_Xp, _Yp>> &&
     is_convertible_v<_Bp&&, __common_ref_C<_Xp, _Yp>>
 struct __common_ref<_Ap&&, _Bp&&, _Xp, _Yp> {
-  using __type = __common_ref_C<_Xp, _Yp>;
+  using __type _LIBCPP_NODEBUG = __common_ref_C<_Xp, _Yp>;
 };
 // clang-format on
 
 //    Otherwise, let D be COMMON-REF(const X&, Y&). ...
 template <class _Tp, class _Up>
-using __common_ref_D = __common_ref_t<const _Tp&, _Up&>;
+using __common_ref_D _LIBCPP_NODEBUG = __common_ref_t<const _Tp&, _Up&>;
 
 //    ... If A is an rvalue reference and B is an lvalue reference and D is well-formed and
 //    is_convertible_v<A, D> is true, then COMMON-REF(A, B) is D.
@@ -94,7 +94,7 @@ template <class _Ap, class _Bp, class _Xp, class _Yp>
     requires { typename __common_ref_D<_Xp, _Yp>; } &&
     is_convertible_v<_Ap&&, __common_ref_D<_Xp, _Yp>>
 struct __common_ref<_Ap&&, _Bp&, _Xp, _Yp> {
-  using __type = __common_ref_D<_Xp, _Yp>;
+  using __type _LIBCPP_NODEBUG = __common_ref_D<_Xp, _Yp>;
 };
 // clang-format on
 
@@ -150,7 +150,7 @@ template <class, class, template <class> class, template <class> class>
 struct basic_common_reference {};
 
 template <class _Tp, class _Up>
-using __basic_common_reference_t =
+using __basic_common_reference_t _LIBCPP_NODEBUG =
     typename basic_common_reference<remove_cvref_t<_Tp>,
                                     remove_cvref_t<_Up>,
                                     __xref<_Tp>::template __apply,
diff --git a/libcxx/include/__type_traits/common_type.h b/libcxx/include/__type_traits/common_type.h
index ee5596bd1249..e4c6b495c3bd 100644
--- a/libcxx/include/__type_traits/common_type.h
+++ b/libcxx/include/__type_traits/common_type.h
@@ -31,7 +31,7 @@ template <class... _Args>
 struct common_type;
 
 template <class... _Args>
-using __common_type_t = typename common_type<_Args...>::type;
+using __common_type_t _LIBCPP_NODEBUG = typename common_type<_Args...>::type;
 
 template <class... _Args>
 struct common_type : __builtin_common_type<__common_type_t, __type_identity, __empty, _Args...> {};
@@ -40,7 +40,7 @@ struct common_type : __builtin_common_type<__common_type_t, __type_identity, __e
 #  if _LIBCPP_STD_VER >= 20
 // Let COND_RES(X, Y) be:
 template <class _Tp, class _Up>
-using __cond_type = decltype(false ? std::declval<_Tp>() : std::declval<_Up>());
+using __cond_type _LIBCPP_NODEBUG = decltype(false ? std::declval<_Tp>() : std::declval<_Up>());
 
 template <class _Tp, class _Up, class = void>
 struct __common_type3 {};
diff --git a/libcxx/include/__type_traits/conjunction.h b/libcxx/include/__type_traits/conjunction.h
index c2995591bbc2..4001d6c12d5d 100644
--- a/libcxx/include/__type_traits/conjunction.h
+++ b/libcxx/include/__type_traits/conjunction.h
@@ -22,7 +22,7 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class...>
-using __expand_to_true = true_type;
+using __expand_to_true _LIBCPP_NODEBUG = true_type;
 
 template <class... _Pred>
 __expand_to_true<__enable_if_t<_Pred::value>...> __and_helper(int);
diff --git a/libcxx/include/__type_traits/copy_cv.h b/libcxx/include/__type_traits/copy_cv.h
index d482cb42bffe..8378fbd50ef5 100644
--- a/libcxx/include/__type_traits/copy_cv.h
+++ b/libcxx/include/__type_traits/copy_cv.h
@@ -22,29 +22,29 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _From>
 struct __copy_cv {
   template <class _To>
-  using __apply = _To;
+  using __apply _LIBCPP_NODEBUG = _To;
 };
 
 template <class _From>
 struct __copy_cv<const _From> {
   template <class _To>
-  using __apply = const _To;
+  using __apply _LIBCPP_NODEBUG = const _To;
 };
 
 template <class _From>
 struct __copy_cv<volatile _From> {
   template <class _To>
-  using __apply = volatile _To;
+  using __apply _LIBCPP_NODEBUG = volatile _To;
 };
 
 template <class _From>
 struct __copy_cv<const volatile _From> {
   template <class _To>
-  using __apply = const volatile _To;
+  using __apply _LIBCPP_NODEBUG = const volatile _To;
 };
 
 template <class _From, class _To>
-using __copy_cv_t = typename __copy_cv<_From>::template __apply<_To>;
+using __copy_cv_t _LIBCPP_NODEBUG = typename __copy_cv<_From>::template __apply<_To>;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__type_traits/copy_cvref.h b/libcxx/include/__type_traits/copy_cvref.h
index 8bbf8efdf44d..511d4e0776d6 100644
--- a/libcxx/include/__type_traits/copy_cvref.h
+++ b/libcxx/include/__type_traits/copy_cvref.h
@@ -36,7 +36,7 @@ struct __copy_cvref<_From&&, _To> {
 };
 
 template <class _From, class _To>
-using __copy_cvref_t = typename __copy_cvref<_From, _To>::type;
+using __copy_cvref_t _LIBCPP_NODEBUG = typename __copy_cvref<_From, _To>::type;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__type_traits/disjunction.h b/libcxx/include/__type_traits/disjunction.h
index 2c89528d9f2f..d579de9b9843 100644
--- a/libcxx/include/__type_traits/disjunction.h
+++ b/libcxx/include/__type_traits/disjunction.h
@@ -31,7 +31,7 @@ struct _OrImpl<true> {
 template <>
 struct _OrImpl<false> {
   template <class _Res, class...>
-  using _Result = _Res;
+  using _Result _LIBCPP_NODEBUG = _Res;
 };
 
 // _Or always performs lazy evaluation of its arguments.
diff --git a/libcxx/include/__type_traits/invoke.h b/libcxx/include/__type_traits/invoke.h
index 71db32ae6a3c..6f641b9a81b8 100644
--- a/libcxx/include/__type_traits/invoke.h
+++ b/libcxx/include/__type_traits/invoke.h
@@ -44,12 +44,12 @@ template <class _Fp,
           class _DecayFp = __decay_t<_Fp>,
           class _DecayA0 = __decay_t<_A0>,
           class _ClassT  = typename __member_pointer_class_type<_DecayFp>::type>
-using __enable_if_bullet1 =
+using __enable_if_bullet1 _LIBCPP_NODEBUG =
     __enable_if_t<is_member_function_pointer<_DecayFp>::value &&
                   (is_same<_ClassT, _DecayA0>::value || is_base_of<_ClassT, _DecayA0>::value)>;
 
 template <class _Fp, class _A0, class _DecayFp = __decay_t<_Fp>, class _DecayA0 = __decay_t<_A0> >
-using __enable_if_bullet2 =
+using __enable_if_bullet2 _LIBCPP_NODEBUG =
     __enable_if_t<is_member_function_pointer<_DecayFp>::value && __is_reference_wrapper<_DecayA0>::value>;
 
 template <class _Fp,
@@ -57,7 +57,7 @@ template <class _Fp,
           class _DecayFp = __decay_t<_Fp>,
           class _DecayA0 = __decay_t<_A0>,
           class _ClassT  = typename __member_pointer_class_type<_DecayFp>::type>
-using __enable_if_bullet3 =
+using __enable_if_bullet3 _LIBCPP_NODEBUG =
     __enable_if_t<is_member_function_pointer<_DecayFp>::value &&
                   !(is_same<_ClassT, _DecayA0>::value || is_base_of<_ClassT, _DecayA0>::value) &&
                   !__is_reference_wrapper<_DecayA0>::value>;
@@ -67,12 +67,12 @@ template <class _Fp,
           class _DecayFp = __decay_t<_Fp>,
           class _DecayA0 = __decay_t<_A0>,
           class _ClassT  = typename __member_pointer_class_type<_DecayFp>::type>
-using __enable_if_bullet4 =
+using __enable_if_bullet4 _LIBCPP_NODEBUG =
     __enable_if_t<is_member_object_pointer<_DecayFp>::value &&
                   (is_same<_ClassT, _DecayA0>::value || is_base_of<_ClassT, _DecayA0>::value)>;
 
 template <class _Fp, class _A0, class _DecayFp = __decay_t<_Fp>, class _DecayA0 = __decay_t<_A0> >
-using __enable_if_bullet5 =
+using __enable_if_bullet5 _LIBCPP_NODEBUG =
     __enable_if_t<is_member_object_pointer<_DecayFp>::value && __is_reference_wrapper<_DecayA0>::value>;
 
 template <class _Fp,
@@ -80,7 +80,7 @@ template <class _Fp,
           class _DecayFp = __decay_t<_Fp>,
           class _DecayA0 = __decay_t<_A0>,
           class _ClassT  = typename __member_pointer_class_type<_DecayFp>::type>
-using __enable_if_bullet6 =
+using __enable_if_bullet6 _LIBCPP_NODEBUG =
     __enable_if_t<is_member_object_pointer<_DecayFp>::value &&
                   !(is_same<_ClassT, _DecayA0>::value || is_base_of<_ClassT, _DecayA0>::value) &&
                   !__is_reference_wrapper<_DecayA0>::value>;
@@ -159,7 +159,7 @@ struct __invokable_r {
 
   // FIXME: Check that _Ret, _Fp, and _Args... are all complete types, cv void,
   // or incomplete array types as required by the standard.
-  using _Result = decltype(__try_call<_Fp, _Args...>(0));
+  using _Result _LIBCPP_NODEBUG = decltype(__try_call<_Fp, _Args...>(0));
 
   using type              = __conditional_t<_IsNotSame<_Result, __nat>::value,
                                             __conditional_t<is_void<_Ret>::value, true_type, __is_core_convertible<_Result, _Ret> >,
@@ -167,7 +167,7 @@ struct __invokable_r {
   static const bool value = type::value;
 };
 template <class _Fp, class... _Args>
-using __invokable = __invokable_r<void, _Fp, _Args...>;
+using __invokable _LIBCPP_NODEBUG = __invokable_r<void, _Fp, _Args...>;
 
 template <bool _IsInvokable, bool _IsCVVoid, class _Ret, class _Fp, class... _Args>
 struct __nothrow_invokable_r_imp {
@@ -199,11 +199,12 @@ struct __nothrow_invokable_r_imp<true, true, _Ret, _Fp, _Args...> {
 };
 
 template <class _Ret, class _Fp, class... _Args>
-using __nothrow_invokable_r =
+using __nothrow_invokable_r _LIBCPP_NODEBUG =
     __nothrow_invokable_r_imp<__invokable_r<_Ret, _Fp, _Args...>::value, is_void<_Ret>::value, _Ret, _Fp, _Args...>;
 
 template <class _Fp, class... _Args>
-using __nothrow_invokable = __nothrow_invokable_r_imp<__invokable<_Fp, _Args...>::value, true, void, _Fp, _Args...>;
+using __nothrow_invokable _LIBCPP_NODEBUG =
+    __nothrow_invokable_r_imp<__invokable<_Fp, _Args...>::value, true, void, _Fp, _Args...>;
 
 template <class _Fp, class... _Args>
 struct __invoke_of
diff --git a/libcxx/include/__type_traits/is_always_bitcastable.h b/libcxx/include/__type_traits/is_always_bitcastable.h
index 5bc650b41358..4c6c43c6571f 100644
--- a/libcxx/include/__type_traits/is_always_bitcastable.h
+++ b/libcxx/include/__type_traits/is_always_bitcastable.h
@@ -31,8 +31,8 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 // considered bit-castable.
 template <class _From, class _To>
 struct __is_always_bitcastable {
-  using _UnqualFrom = __remove_cv_t<_From>;
-  using _UnqualTo   = __remove_cv_t<_To>;
+  using _UnqualFrom _LIBCPP_NODEBUG = __remove_cv_t<_From>;
+  using _UnqualTo _LIBCPP_NODEBUG   = __remove_cv_t<_To>;
 
   // clang-format off
   static const bool value =
diff --git a/libcxx/include/__type_traits/is_char_like_type.h b/libcxx/include/__type_traits/is_char_like_type.h
index 26205843047c..913c0821c8c6 100644
--- a/libcxx/include/__type_traits/is_char_like_type.h
+++ b/libcxx/include/__type_traits/is_char_like_type.h
@@ -21,7 +21,7 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _CharT>
-using _IsCharLikeType = _And<is_standard_layout<_CharT>, is_trivial<_CharT> >;
+using _IsCharLikeType _LIBCPP_NODEBUG = _And<is_standard_layout<_CharT>, is_trivial<_CharT> >;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__type_traits/is_equality_comparable.h b/libcxx/include/__type_traits/is_equality_comparable.h
index 4397f743e5ee..3ee1839996be 100644
--- a/libcxx/include/__type_traits/is_equality_comparable.h
+++ b/libcxx/include/__type_traits/is_equality_comparable.h
@@ -80,7 +80,7 @@ struct __libcpp_is_trivially_equality_comparable_impl<_Tp*, _Up*>
 };
 
 template <class _Tp, class _Up>
-using __libcpp_is_trivially_equality_comparable =
+using __libcpp_is_trivially_equality_comparable _LIBCPP_NODEBUG =
     __libcpp_is_trivially_equality_comparable_impl<__remove_cv_t<_Tp>, __remove_cv_t<_Up> >;
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/__type_traits/is_execution_policy.h b/libcxx/include/__type_traits/is_execution_policy.h
index 6884f17ba16c..a2d876db0309 100644
--- a/libcxx/include/__type_traits/is_execution_policy.h
+++ b/libcxx/include/__type_traits/is_execution_policy.h
@@ -50,7 +50,7 @@ __remove_parallel_policy(const _ExecutionPolicy& = _ExecutionPolicy{execution::_
 // Removes the "parallel" part of an execution policy.
 // For example, turns par_unseq into unseq, and par into seq.
 template <class _ExecutionPolicy>
-using __remove_parallel_policy_t = decltype(std::__remove_parallel_policy<_ExecutionPolicy>());
+using __remove_parallel_policy_t _LIBCPP_NODEBUG = decltype(std::__remove_parallel_policy<_ExecutionPolicy>());
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__type_traits/is_primary_template.h b/libcxx/include/__type_traits/is_primary_template.h
index f308dfadc8ec..5fe6820bc7f7 100644
--- a/libcxx/include/__type_traits/is_primary_template.h
+++ b/libcxx/include/__type_traits/is_primary_template.h
@@ -21,10 +21,11 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp>
-using __test_for_primary_template = __enable_if_t<_IsSame<_Tp, typename _Tp::__primary_template>::value>;
+using __test_for_primary_template _LIBCPP_NODEBUG =
+    __enable_if_t<_IsSame<_Tp, typename _Tp::__primary_template>::value>;
 
 template <class _Tp>
-using __is_primary_template = _IsValidExpansion<__test_for_primary_template, _Tp>;
+using __is_primary_template _LIBCPP_NODEBUG = _IsValidExpansion<__test_for_primary_template, _Tp>;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__type_traits/is_same.h b/libcxx/include/__type_traits/is_same.h
index 9561b7b5d6da..400f870904d2 100644
--- a/libcxx/include/__type_traits/is_same.h
+++ b/libcxx/include/__type_traits/is_same.h
@@ -34,10 +34,10 @@ inline constexpr bool is_same_v = __is_same(_Tp, _Up);
 // (such as in a dependent return type).
 
 template <class _Tp, class _Up>
-using _IsSame = _BoolConstant<__is_same(_Tp, _Up)>;
+using _IsSame _LIBCPP_NODEBUG = _BoolConstant<__is_same(_Tp, _Up)>;
 
 template <class _Tp, class _Up>
-using _IsNotSame = _BoolConstant<!__is_same(_Tp, _Up)>;
+using _IsNotSame _LIBCPP_NODEBUG = _BoolConstant<!__is_same(_Tp, _Up)>;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__type_traits/is_swappable.h b/libcxx/include/__type_traits/is_swappable.h
index 221f017700a2..aa5eecd9abe0 100644
--- a/libcxx/include/__type_traits/is_swappable.h
+++ b/libcxx/include/__type_traits/is_swappable.h
@@ -41,10 +41,11 @@ inline const bool __is_nothrow_swappable_v = __is_nothrow_swappable_with_v<_Tp&,
 
 #ifndef _LIBCPP_CXX03_LANG
 template <class _Tp>
-using __swap_result_t = __enable_if_t<is_move_constructible<_Tp>::value && is_move_assignable<_Tp>::value>;
+using __swap_result_t _LIBCPP_NODEBUG =
+    __enable_if_t<is_move_constructible<_Tp>::value && is_move_assignable<_Tp>::value>;
 #else
 template <class>
-using __swap_result_t = void;
+using __swap_result_t _LIBCPP_NODEBUG = void;
 #endif
 
 template <class _Tp>
diff --git a/libcxx/include/__type_traits/make_32_64_or_128_bit.h b/libcxx/include/__type_traits/make_32_64_or_128_bit.h
index 70f84fcd1868..7016209ec9c0 100644
--- a/libcxx/include/__type_traits/make_32_64_or_128_bit.h
+++ b/libcxx/include/__type_traits/make_32_64_or_128_bit.h
@@ -31,7 +31,7 @@ template <class _Tp>
   requires(is_signed_v<_Tp> || is_unsigned_v<_Tp> || is_same_v<_Tp, char>)
 #endif
 // clang-format off
-using __make_32_64_or_128_bit_t =
+using __make_32_64_or_128_bit_t _LIBCPP_NODEBUG =
     __copy_unsigned_t<_Tp,
         __conditional_t<sizeof(_Tp) <= sizeof(int32_t),    int32_t,
         __conditional_t<sizeof(_Tp) <= sizeof(int64_t),    int64_t,
diff --git a/libcxx/include/__type_traits/make_const_lvalue_ref.h b/libcxx/include/__type_traits/make_const_lvalue_ref.h
index 469d4cb31ef7..f9955334de30 100644
--- a/libcxx/include/__type_traits/make_const_lvalue_ref.h
+++ b/libcxx/include/__type_traits/make_const_lvalue_ref.h
@@ -19,7 +19,7 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp>
-using __make_const_lvalue_ref = const __libcpp_remove_reference_t<_Tp>&;
+using __make_const_lvalue_ref _LIBCPP_NODEBUG = const __libcpp_remove_reference_t<_Tp>&;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__type_traits/make_signed.h b/libcxx/include/__type_traits/make_signed.h
index 8070690b3a7a..88513fea3006 100644
--- a/libcxx/include/__type_traits/make_signed.h
+++ b/libcxx/include/__type_traits/make_signed.h
@@ -26,24 +26,20 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #if __has_builtin(__make_signed)
 
 template <class _Tp>
-using __make_signed_t = __make_signed(_Tp);
+using __make_signed_t _LIBCPP_NODEBUG = __make_signed(_Tp);
 
 #else
-// clang-format off
-typedef __type_list<signed char,
-        __type_list<signed short,
-        __type_list<signed int,
-        __type_list<signed long,
-        __type_list<signed long long,
-#  if _LIBCPP_HAS_INT128
-        __type_list<__int128_t,
-#  endif
-        __nat
+using __signed_types =
+    __type_list<signed char,
+                signed short,
+                signed int,
+                signed long,
+                signed long long
 #  if _LIBCPP_HAS_INT128
-        >
+                ,
+                __int128_t
 #  endif
-        > > > > > __signed_types;
-// clang-format on
+                >;
 
 template <class _Tp, bool = is_integral<_Tp>::value || is_enum<_Tp>::value>
 struct __make_signed{};
diff --git a/libcxx/include/__type_traits/make_unsigned.h b/libcxx/include/__type_traits/make_unsigned.h
index 562f7bab8a7f..83ff8b7bb801 100644
--- a/libcxx/include/__type_traits/make_unsigned.h
+++ b/libcxx/include/__type_traits/make_unsigned.h
@@ -28,24 +28,20 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #if __has_builtin(__make_unsigned)
 
 template <class _Tp>
-using __make_unsigned_t = __make_unsigned(_Tp);
+using __make_unsigned_t _LIBCPP_NODEBUG = __make_unsigned(_Tp);
 
 #else
-// clang-format off
-typedef __type_list<unsigned char,
-        __type_list<unsigned short,
-        __type_list<unsigned int,
-        __type_list<unsigned long,
-        __type_list<unsigned long long,
-#  if _LIBCPP_HAS_INT128
-        __type_list<__uint128_t,
-#  endif
-        __nat
+using __unsigned_types =
+    __type_list<unsigned char,
+                unsigned short,
+                unsigned int,
+                unsigned long,
+                unsigned long long
 #  if _LIBCPP_HAS_INT128
-        >
+                ,
+                __uint128_t
 #  endif
-        > > > > > __unsigned_types;
-// clang-format on
+                >;
 
 template <class _Tp, bool = is_integral<_Tp>::value || is_enum<_Tp>::value>
 struct __make_unsigned{};
@@ -92,7 +88,7 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __make_unsigned_t<_Tp> __to_unsigned_lik
 }
 
 template <class _Tp, class _Up>
-using __copy_unsigned_t = __conditional_t<is_unsigned<_Tp>::value, __make_unsigned_t<_Up>, _Up>;
+using __copy_unsigned_t _LIBCPP_NODEBUG = __conditional_t<is_unsigned<_Tp>::value, __make_unsigned_t<_Up>, _Up>;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__type_traits/maybe_const.h b/libcxx/include/__type_traits/maybe_const.h
index 25fba58fb773..7ef742a123d0 100644
--- a/libcxx/include/__type_traits/maybe_const.h
+++ b/libcxx/include/__type_traits/maybe_const.h
@@ -19,7 +19,7 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <bool _Const, class _Tp>
-using __maybe_const = __conditional_t<_Const, const _Tp, _Tp>;
+using __maybe_const _LIBCPP_NODEBUG = __conditional_t<_Const, const _Tp, _Tp>;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__type_traits/remove_all_extents.h b/libcxx/include/__type_traits/remove_all_extents.h
index db7dab4a6c13..d46a3228b4ab 100644
--- a/libcxx/include/__type_traits/remove_all_extents.h
+++ b/libcxx/include/__type_traits/remove_all_extents.h
@@ -25,7 +25,7 @@ struct remove_all_extents {
 };
 
 template <class _Tp>
-using __remove_all_extents_t = __remove_all_extents(_Tp);
+using __remove_all_extents_t _LIBCPP_NODEBUG = __remove_all_extents(_Tp);
 #else
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS remove_all_extents {
diff --git a/libcxx/include/__type_traits/remove_const.h b/libcxx/include/__type_traits/remove_const.h
index a3f0648c4785..6250d9f53117 100644
--- a/libcxx/include/__type_traits/remove_const.h
+++ b/libcxx/include/__type_traits/remove_const.h
@@ -24,7 +24,7 @@ struct remove_const {
 };
 
 template <class _Tp>
-using __remove_const_t = __remove_const(_Tp);
+using __remove_const_t _LIBCPP_NODEBUG = __remove_const(_Tp);
 #else
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS remove_const {
diff --git a/libcxx/include/__type_traits/remove_const_ref.h b/libcxx/include/__type_traits/remove_const_ref.h
index d3b334935a5b..e6583b396e6c 100644
--- a/libcxx/include/__type_traits/remove_const_ref.h
+++ b/libcxx/include/__type_traits/remove_const_ref.h
@@ -20,7 +20,7 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp>
-using __remove_const_ref_t = __remove_const_t<__libcpp_remove_reference_t<_Tp> >;
+using __remove_const_ref_t _LIBCPP_NODEBUG = __remove_const_t<__libcpp_remove_reference_t<_Tp> >;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__type_traits/remove_cv.h b/libcxx/include/__type_traits/remove_cv.h
index 50e9f3e8aa78..16848e6d7112 100644
--- a/libcxx/include/__type_traits/remove_cv.h
+++ b/libcxx/include/__type_traits/remove_cv.h
@@ -24,10 +24,10 @@ struct remove_cv {
 
 #if defined(_LIBCPP_COMPILER_GCC)
 template <class _Tp>
-using __remove_cv_t = typename remove_cv<_Tp>::type;
+using __remove_cv_t _LIBCPP_NODEBUG = typename remove_cv<_Tp>::type;
 #else
 template <class _Tp>
-using __remove_cv_t = __remove_cv(_Tp);
+using __remove_cv_t _LIBCPP_NODEBUG = __remove_cv(_Tp);
 #endif
 
 #if _LIBCPP_STD_VER >= 14
diff --git a/libcxx/include/__type_traits/remove_cvref.h b/libcxx/include/__type_traits/remove_cvref.h
index 55f894dbd1d8..e3c65944e33c 100644
--- a/libcxx/include/__type_traits/remove_cvref.h
+++ b/libcxx/include/__type_traits/remove_cvref.h
@@ -34,7 +34,7 @@ using __remove_cvref_t _LIBCPP_NODEBUG = __remove_cvref(_Tp);
 #endif // __has_builtin(__remove_cvref)
 
 template <class _Tp, class _Up>
-using __is_same_uncvref = _IsSame<__remove_cvref_t<_Tp>, __remove_cvref_t<_Up> >;
+using __is_same_uncvref _LIBCPP_NODEBUG = _IsSame<__remove_cvref_t<_Tp>, __remove_cvref_t<_Up> >;
 
 #if _LIBCPP_STD_VER >= 20
 template <class _Tp>
diff --git a/libcxx/include/__type_traits/remove_extent.h b/libcxx/include/__type_traits/remove_extent.h
index aceeb4706966..95a7971d7a9c 100644
--- a/libcxx/include/__type_traits/remove_extent.h
+++ b/libcxx/include/__type_traits/remove_extent.h
@@ -25,7 +25,7 @@ struct remove_extent {
 };
 
 template <class _Tp>
-using __remove_extent_t = __remove_extent(_Tp);
+using __remove_extent_t _LIBCPP_NODEBUG = __remove_extent(_Tp);
 #else
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS remove_extent {
diff --git a/libcxx/include/__type_traits/remove_pointer.h b/libcxx/include/__type_traits/remove_pointer.h
index 6f98ed1a5d64..47cd1cd1d80f 100644
--- a/libcxx/include/__type_traits/remove_pointer.h
+++ b/libcxx/include/__type_traits/remove_pointer.h
@@ -25,10 +25,10 @@ struct remove_pointer {
 
 #  ifdef _LIBCPP_COMPILER_GCC
 template <class _Tp>
-using __remove_pointer_t = typename remove_pointer<_Tp>::type;
+using __remove_pointer_t _LIBCPP_NODEBUG = typename remove_pointer<_Tp>::type;
 #  else
 template <class _Tp>
-using __remove_pointer_t = __remove_pointer(_Tp);
+using __remove_pointer_t _LIBCPP_NODEBUG = __remove_pointer(_Tp);
 #  endif
 #else
 // clang-format off
diff --git a/libcxx/include/__type_traits/remove_reference.h b/libcxx/include/__type_traits/remove_reference.h
index ba67891758ad..f68815691ac0 100644
--- a/libcxx/include/__type_traits/remove_reference.h
+++ b/libcxx/include/__type_traits/remove_reference.h
@@ -24,7 +24,7 @@ struct remove_reference {
 };
 
 template <class _Tp>
-using __libcpp_remove_reference_t = __remove_reference_t(_Tp);
+using __libcpp_remove_reference_t _LIBCPP_NODEBUG = __remove_reference_t(_Tp);
 #elif __has_builtin(__remove_reference)
 template <class _Tp>
 struct remove_reference {
diff --git a/libcxx/include/__type_traits/remove_volatile.h b/libcxx/include/__type_traits/remove_volatile.h
index 7600ae0ec516..099945df0124 100644
--- a/libcxx/include/__type_traits/remove_volatile.h
+++ b/libcxx/include/__type_traits/remove_volatile.h
@@ -24,7 +24,7 @@ struct remove_volatile {
 };
 
 template <class _Tp>
-using __remove_volatile_t = __remove_volatile(_Tp);
+using __remove_volatile_t _LIBCPP_NODEBUG = __remove_volatile(_Tp);
 #else
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS remove_volatile {
diff --git a/libcxx/include/__type_traits/type_list.h b/libcxx/include/__type_traits/type_list.h
index b4898b36e2d9..34d78fc97c97 100644
--- a/libcxx/include/__type_traits/type_list.h
+++ b/libcxx/include/__type_traits/type_list.h
@@ -11,6 +11,7 @@
 
 #include <__config>
 #include <__cstddef/size_t.h>
+#include <__type_traits/enable_if.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -18,23 +19,28 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _Hp, class _Tp>
-struct __type_list {
-  typedef _Hp _Head;
-  typedef _Tp _Tail;
+template <class... _Types>
+struct __type_list {};
+
+template <class>
+struct __type_list_head;
+
+template <class _Head, class... _Tail>
+struct __type_list_head<__type_list<_Head, _Tail...> > {
+  using type _LIBCPP_NODEBUG = _Head;
 };
 
-template <class _TypeList, size_t _Size, bool = _Size <= sizeof(typename _TypeList::_Head)>
+template <class _TypeList, size_t _Size, bool = _Size <= sizeof(typename __type_list_head<_TypeList>::type)>
 struct __find_first;
 
-template <class _Hp, class _Tp, size_t _Size>
-struct __find_first<__type_list<_Hp, _Tp>, _Size, true> {
-  using type _LIBCPP_NODEBUG = _Hp;
+template <class _Head, class... _Tail, size_t _Size>
+struct __find_first<__type_list<_Head, _Tail...>, _Size, true> {
+  using type _LIBCPP_NODEBUG = _Head;
 };
 
-template <class _Hp, class _Tp, size_t _Size>
-struct __find_first<__type_list<_Hp, _Tp>, _Size, false> {
-  using type _LIBCPP_NODEBUG = typename __find_first<_Tp, _Size>::type;
+template <class _Head, class... _Tail, size_t _Size>
+struct __find_first<__type_list<_Head, _Tail...>, _Size, false> {
+  using type _LIBCPP_NODEBUG = typename __find_first<__type_list<_Tail...>, _Size>::type;
 };
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/__type_traits/unwrap_ref.h b/libcxx/include/__type_traits/unwrap_ref.h
index 74c4fde915c3..11a069d66302 100644
--- a/libcxx/include/__type_traits/unwrap_ref.h
+++ b/libcxx/include/__type_traits/unwrap_ref.h
@@ -29,6 +29,9 @@ struct __unwrap_reference<reference_wrapper<_Tp> > {
   using type _LIBCPP_NODEBUG = _Tp&;
 };
 
+template <class _Tp>
+using __unwrap_ref_decay_t _LIBCPP_NODEBUG = typename __unwrap_reference<__decay_t<_Tp> >::type;
+
 #if _LIBCPP_STD_VER >= 20
 template <class _Tp>
 struct unwrap_reference : __unwrap_reference<_Tp> {};
@@ -40,19 +43,9 @@ template <class _Tp>
 struct unwrap_ref_decay : unwrap_reference<__decay_t<_Tp> > {};
 
 template <class _Tp>
-using unwrap_ref_decay_t = typename unwrap_ref_decay<_Tp>::type;
+using unwrap_ref_decay_t = __unwrap_ref_decay_t<_Tp>;
 #endif // _LIBCPP_STD_VER >= 20
 
-template <class _Tp>
-struct __unwrap_ref_decay
-#if _LIBCPP_STD_VER >= 20
-    : unwrap_ref_decay<_Tp>
-#else
-    : __unwrap_reference<__decay_t<_Tp> >
-#endif
-{
-};
-
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___TYPE_TRAITS_UNWRAP_REF_H
diff --git a/libcxx/include/__type_traits/void_t.h b/libcxx/include/__type_traits/void_t.h
index 985bba02e72f..8adadfa69637 100644
--- a/libcxx/include/__type_traits/void_t.h
+++ b/libcxx/include/__type_traits/void_t.h
@@ -23,7 +23,7 @@ using void_t = void;
 #endif
 
 template <class...>
-using __void_t = void;
+using __void_t _LIBCPP_NODEBUG = void;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/__utility/exception_guard.h b/libcxx/include/__utility/exception_guard.h
index 71e52fdb4b2a..a6b4ec521107 100644
--- a/libcxx/include/__utility/exception_guard.h
+++ b/libcxx/include/__utility/exception_guard.h
@@ -126,10 +126,10 @@ _LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_noexceptions);
 
 #if !_LIBCPP_HAS_EXCEPTIONS
 template <class _Rollback>
-using __exception_guard = __exception_guard_noexceptions<_Rollback>;
+using __exception_guard _LIBCPP_NODEBUG = __exception_guard_noexceptions<_Rollback>;
 #else
 template <class _Rollback>
-using __exception_guard = __exception_guard_exceptions<_Rollback>;
+using __exception_guard _LIBCPP_NODEBUG = __exception_guard_exceptions<_Rollback>;
 #endif
 
 template <class _Rollback>
diff --git a/libcxx/include/__utility/forward_like.h b/libcxx/include/__utility/forward_like.h
index 67bdf6d054ea..409f716cfbce 100644
--- a/libcxx/include/__utility/forward_like.h
+++ b/libcxx/include/__utility/forward_like.h
@@ -26,13 +26,13 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #if _LIBCPP_STD_VER >= 23
 
 template <class _Ap, class _Bp>
-using _CopyConst = _If<is_const_v<_Ap>, const _Bp, _Bp>;
+using _CopyConst _LIBCPP_NODEBUG = _If<is_const_v<_Ap>, const _Bp, _Bp>;
 
 template <class _Ap, class _Bp>
-using _OverrideRef = _If<is_rvalue_reference_v<_Ap>, remove_reference_t<_Bp>&&, _Bp&>;
+using _OverrideRef _LIBCPP_NODEBUG = _If<is_rvalue_reference_v<_Ap>, remove_reference_t<_Bp>&&, _Bp&>;
 
 template <class _Ap, class _Bp>
-using _ForwardLike = _OverrideRef<_Ap&&, _CopyConst<remove_reference_t<_Ap>, remove_reference_t<_Bp>>>;
+using _ForwardLike _LIBCPP_NODEBUG = _OverrideRef<_Ap&&, _CopyConst<remove_reference_t<_Ap>, remove_reference_t<_Bp>>>;
 
 template <class _Tp, class _Up>
 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto
diff --git a/libcxx/include/__utility/in_place.h b/libcxx/include/__utility/in_place.h
index edaa4e02c55f..9b48446d8370 100644
--- a/libcxx/include/__utility/in_place.h
+++ b/libcxx/include/__utility/in_place.h
@@ -47,7 +47,7 @@ template <class _Tp>
 struct __is_inplace_type_imp<in_place_type_t<_Tp>> : true_type {};
 
 template <class _Tp>
-using __is_inplace_type = __is_inplace_type_imp<__remove_cvref_t<_Tp>>;
+using __is_inplace_type _LIBCPP_NODEBUG = __is_inplace_type_imp<__remove_cvref_t<_Tp>>;
 
 template <class _Tp>
 struct __is_inplace_index_imp : false_type {};
@@ -55,7 +55,7 @@ template <size_t _Idx>
 struct __is_inplace_index_imp<in_place_index_t<_Idx>> : true_type {};
 
 template <class _Tp>
-using __is_inplace_index = __is_inplace_index_imp<__remove_cvref_t<_Tp>>;
+using __is_inplace_index _LIBCPP_NODEBUG = __is_inplace_index_imp<__remove_cvref_t<_Tp>>;
 
 #endif // _LIBCPP_STD_VER >= 17
 
diff --git a/libcxx/include/__utility/integer_sequence.h b/libcxx/include/__utility/integer_sequence.h
index 35eb606ee37f..2c1ff3c543e8 100644
--- a/libcxx/include/__utility/integer_sequence.h
+++ b/libcxx/include/__utility/integer_sequence.h
@@ -25,19 +25,19 @@ struct __tuple_indices;
 template <class _IdxType, _IdxType... _Values>
 struct __integer_sequence {
   template <template <class _OIdxType, _OIdxType...> class _ToIndexSeq, class _ToIndexType>
-  using __convert = _ToIndexSeq<_ToIndexType, _Values...>;
+  using __convert _LIBCPP_NODEBUG = _ToIndexSeq<_ToIndexType, _Values...>;
 
   template <size_t _Sp>
-  using __to_tuple_indices = __tuple_indices<(_Values + _Sp)...>;
+  using __to_tuple_indices _LIBCPP_NODEBUG = __tuple_indices<(_Values + _Sp)...>;
 };
 
 #if __has_builtin(__make_integer_seq)
 template <size_t _Ep, size_t _Sp>
-using __make_indices_imp =
+using __make_indices_imp _LIBCPP_NODEBUG =
     typename __make_integer_seq<__integer_sequence, size_t, _Ep - _Sp>::template __to_tuple_indices<_Sp>;
 #elif __has_builtin(__integer_pack)
 template <size_t _Ep, size_t _Sp>
-using __make_indices_imp =
+using __make_indices_imp _LIBCPP_NODEBUG =
     typename __integer_sequence<size_t, __integer_pack(_Ep - _Sp)...>::template __to_tuple_indices<_Sp>;
 #else
 #  error "No known way to get an integer pack from the compiler"
diff --git a/libcxx/include/__utility/move.h b/libcxx/include/__utility/move.h
index 015986f610bd..bc16697b5c89 100644
--- a/libcxx/include/__utility/move.h
+++ b/libcxx/include/__utility/move.h
@@ -33,7 +33,7 @@ move(_LIBCPP_LIFETIMEBOUND _Tp&& __t) _NOEXCEPT {
 }
 
 template <class _Tp>
-using __move_if_noexcept_result_t =
+using __move_if_noexcept_result_t _LIBCPP_NODEBUG =
     __conditional_t<!is_nothrow_move_constructible<_Tp>::value && is_copy_constructible<_Tp>::value, const _Tp&, _Tp&&>;
 
 template <class _Tp>
diff --git a/libcxx/include/__utility/pair.h b/libcxx/include/__utility/pair.h
index f9d0f4e47231..7689ab2a48c6 100644
--- a/libcxx/include/__utility/pair.h
+++ b/libcxx/include/__utility/pair.h
@@ -71,7 +71,7 @@ struct _LIBCPP_TEMPLATE_VIS pair
   _T1 first;
   _T2 second;
 
-  using __trivially_relocatable =
+  using __trivially_relocatable _LIBCPP_NODEBUG =
       __conditional_t<__libcpp_is_trivially_relocatable<_T1>::value && __libcpp_is_trivially_relocatable<_T2>::value,
                       pair,
                       void>;
@@ -532,11 +532,9 @@ swap(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y) noexcept(noexcept(__x
 #endif
 
 template <class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI
-_LIBCPP_CONSTEXPR_SINCE_CXX14 pair<typename __unwrap_ref_decay<_T1>::type, typename __unwrap_ref_decay<_T2>::type>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<__unwrap_ref_decay_t<_T1>, __unwrap_ref_decay_t<_T2> >
 make_pair(_T1&& __t1, _T2&& __t2) {
-  return pair<typename __unwrap_ref_decay<_T1>::type, typename __unwrap_ref_decay<_T2>::type>(
-      std::forward<_T1>(__t1), std::forward<_T2>(__t2));
+  return pair<__unwrap_ref_decay_t<_T1>, __unwrap_ref_decay_t<_T2> >(std::forward<_T1>(__t1), std::forward<_T2>(__t2));
 }
 
 template <class _T1, class _T2>
diff --git a/libcxx/include/__utility/swap.h b/libcxx/include/__utility/swap.h
index 666d6d50f0d9..b4311540d36e 100644
--- a/libcxx/include/__utility/swap.h
+++ b/libcxx/include/__utility/swap.h
@@ -31,10 +31,11 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #ifndef _LIBCPP_CXX03_LANG
 template <class _Tp>
-using __swap_result_t = __enable_if_t<is_move_constructible<_Tp>::value && is_move_assignable<_Tp>::value>;
+using __swap_result_t _LIBCPP_NODEBUG =
+    __enable_if_t<is_move_constructible<_Tp>::value && is_move_assignable<_Tp>::value>;
 #else
 template <class>
-using __swap_result_t = void;
+using __swap_result_t _LIBCPP_NODEBUG = void;
 #endif
 
 template <class _Tp>
diff --git a/libcxx/include/__vector/vector.h b/libcxx/include/__vector/vector.h
index 6ba7ba7bcf72..ddbf1235b906 100644
--- a/libcxx/include/__vector/vector.h
+++ b/libcxx/include/__vector/vector.h
@@ -114,7 +114,7 @@ public:
   // - pointer: may be trivially relocatable, so it's checked
   // - allocator_type: may be trivially relocatable, so it's checked
   // vector doesn't contain any self-references, so it's trivially relocatable if its members are.
-  using __trivially_relocatable = __conditional_t<
+  using __trivially_relocatable _LIBCPP_NODEBUG = __conditional_t<
       __libcpp_is_trivially_relocatable<pointer>::value && __libcpp_is_trivially_relocatable<allocator_type>::value,
       vector,
       void>;
diff --git a/libcxx/include/__vector/vector_bool.h b/libcxx/include/__vector/vector_bool.h
index 525fc35b26cc..6c6605fb3bd0 100644
--- a/libcxx/include/__vector/vector_bool.h
+++ b/libcxx/include/__vector/vector_bool.h
@@ -275,17 +275,33 @@ public:
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reverse_iterator crend() const _NOEXCEPT { return rend(); }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference operator[](size_type __n) { return __make_ref(__n); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference operator[](size_type __n) {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__n < size(), "vector<bool>::operator[] index out of bounds");
+    return __make_ref(__n);
+  }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference operator[](size_type __n) const {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__n < size(), "vector<bool>::operator[] index out of bounds");
     return __make_ref(__n);
   }
-  _LIBCPP_HIDE_FROM_ABI reference at(size_type __n);
-  _LIBCPP_HIDE_FROM_ABI const_reference at(size_type __n) const;
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference at(size_type __n);
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference at(size_type __n) const;
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference front() { return __make_ref(0); }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference front() const { return __make_ref(0); }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference back() { return __make_ref(__size_ - 1); }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference back() const { return __make_ref(__size_ - 1); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference front() {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "vector<bool>::front() called on an empty vector");
+    return __make_ref(0);
+  }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference front() const {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "vector<bool>::front() called on an empty vector");
+    return __make_ref(0);
+  }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference back() {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "vector<bool>::back() called on an empty vector");
+    return __make_ref(__size_ - 1);
+  }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference back() const {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "vector<bool>::back() called on an empty vector");
+    return __make_ref(__size_ - 1);
+  }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void push_back(const value_type& __x);
 #if _LIBCPP_STD_VER >= 14
@@ -310,7 +326,10 @@ public:
   }
 #endif
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void pop_back() { --__size_; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void pop_back() {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "vector<bool>::pop_back called on an empty vector");
+    --__size_;
+  }
 
 #if _LIBCPP_STD_VER >= 14
   template <class... _Args>
@@ -442,7 +461,6 @@ private:
   template <class _InputIterator, class _Sentinel>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
   __construct_at_end(_InputIterator __first, _Sentinel __last, size_type __n);
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __append(size_type __n, const_reference __x);
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference __make_ref(size_type __pos) _NOEXCEPT {
     return reference(__begin_ + __pos / __bits_per_word, __storage_type(1) << __pos % __bits_per_word);
   }
@@ -854,14 +872,15 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void vector<bool, _Allocator>::shrink_to_fit() _NO
 }
 
 template <class _Allocator>
-typename vector<bool, _Allocator>::reference vector<bool, _Allocator>::at(size_type __n) {
+_LIBCPP_CONSTEXPR_SINCE_CXX20 typename vector<bool, _Allocator>::reference vector<bool, _Allocator>::at(size_type __n) {
   if (__n >= size())
     this->__throw_out_of_range();
   return (*this)[__n];
 }
 
 template <class _Allocator>
-typename vector<bool, _Allocator>::const_reference vector<bool, _Allocator>::at(size_type __n) const {
+_LIBCPP_CONSTEXPR_SINCE_CXX20 typename vector<bool, _Allocator>::const_reference
+vector<bool, _Allocator>::at(size_type __n) const {
   if (__n >= size())
     this->__throw_out_of_range();
   return (*this)[__n];
@@ -995,6 +1014,8 @@ vector<bool, _Allocator>::__insert_with_size(
 template <class _Allocator>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 typename vector<bool, _Allocator>::iterator
 vector<bool, _Allocator>::erase(const_iterator __position) {
+  _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+      __position != end(), "vector<bool>::erase(iterator) called with a non-dereferenceable iterator");
   iterator __r = __const_iterator_cast(__position);
   std::copy(__position + 1, this->cend(), __r);
   --__size_;
@@ -1004,6 +1025,8 @@ vector<bool, _Allocator>::erase(const_iterator __position) {
 template <class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 typename vector<bool, _Allocator>::iterator
 vector<bool, _Allocator>::erase(const_iterator __first, const_iterator __last) {
+  _LIBCPP_ASSERT_VALID_INPUT_RANGE(
+      __first <= __last, "vector<bool>::erase(iterator, iterator) called with an invalid range");
   iterator __r        = __const_iterator_cast(__first);
   difference_type __d = __last - __first;
   std::copy(__last, this->cend(), __r);
diff --git a/libcxx/include/any b/libcxx/include/any
index 934c4dbd45a6..786e86b5ccd8 100644
--- a/libcxx/include/any
+++ b/libcxx/include/any
@@ -149,11 +149,11 @@ _LIBCPP_HIDE_FROM_ABI add_pointer_t<_ValueType> any_cast(any*) _NOEXCEPT;
 
 namespace __any_imp {
 _LIBCPP_SUPPRESS_DEPRECATED_PUSH
-using _Buffer = aligned_storage_t<3 * sizeof(void*), alignof(void*)>;
+using _Buffer _LIBCPP_NODEBUG = aligned_storage_t<3 * sizeof(void*), alignof(void*)>;
 _LIBCPP_SUPPRESS_DEPRECATED_POP
 
 template <class _Tp>
-using _IsSmallObject =
+using _IsSmallObject _LIBCPP_NODEBUG =
     integral_constant<bool,
                       sizeof(_Tp) <= sizeof(_Buffer) && alignof(_Buffer) % alignof(_Tp) == 0 &&
                           is_nothrow_move_constructible<_Tp>::value >;
@@ -185,7 +185,7 @@ inline _LIBCPP_HIDE_FROM_ABI bool __compare_typeid(type_info const* __id, const
 }
 
 template <class _Tp>
-using _Handler = conditional_t< _IsSmallObject<_Tp>::value, _SmallHandler<_Tp>, _LargeHandler<_Tp>>;
+using _Handler _LIBCPP_NODEBUG = conditional_t< _IsSmallObject<_Tp>::value, _SmallHandler<_Tp>, _LargeHandler<_Tp>>;
 
 } // namespace __any_imp
 
@@ -278,8 +278,9 @@ public:
 #    endif
 
 private:
-  typedef __any_imp::_Action _Action;
-  using _HandleFuncPtr = void* (*)(_Action, any const*, any*, const type_info*, const void* __fallback_info);
+  using _Action _LIBCPP_NODEBUG = __any_imp::_Action;
+  using _HandleFuncPtr
+      _LIBCPP_NODEBUG = void* (*)(_Action, any const*, any*, const type_info*, const void* __fallback_info);
 
   union _Storage {
     _LIBCPP_HIDE_FROM_ABI constexpr _Storage() : __ptr(nullptr) {}
diff --git a/libcxx/include/array b/libcxx/include/array
index 516d96538f5a..1b9bcd6891d9 100644
--- a/libcxx/include/array
+++ b/libcxx/include/array
@@ -173,15 +173,16 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp, size_t _Size>
 struct _LIBCPP_TEMPLATE_VIS array {
-  using __trivially_relocatable = __conditional_t<__libcpp_is_trivially_relocatable<_Tp>::value, array, void>;
+  using __trivially_relocatable _LIBCPP_NODEBUG =
+      __conditional_t<__libcpp_is_trivially_relocatable<_Tp>::value, array, void>;
 
   // types:
-  using __self          = array;
-  using value_type      = _Tp;
-  using reference       = value_type&;
-  using const_reference = const value_type&;
-  using pointer         = value_type*;
-  using const_pointer   = const value_type*;
+  using __self _LIBCPP_NODEBUG = array;
+  using value_type             = _Tp;
+  using reference              = value_type&;
+  using const_reference        = const value_type&;
+  using pointer                = value_type*;
+  using const_pointer          = const value_type*;
 #  if defined(_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY)
   using iterator       = __static_bounded_iter<pointer, _Size>;
   using const_iterator = __static_bounded_iter<const_pointer, _Size>;
@@ -299,12 +300,12 @@ struct _LIBCPP_TEMPLATE_VIS array {
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS array<_Tp, 0> {
   // types:
-  using __self          = array;
-  using value_type      = _Tp;
-  using reference       = value_type&;
-  using const_reference = const value_type&;
-  using pointer         = value_type*;
-  using const_pointer   = const value_type*;
+  using __self _LIBCPP_NODEBUG = array;
+  using value_type             = _Tp;
+  using reference              = value_type&;
+  using const_reference        = const value_type&;
+  using pointer                = value_type*;
+  using const_pointer          = const value_type*;
 #  if defined(_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY)
   using iterator       = __static_bounded_iter<pointer, 0>;
   using const_iterator = __static_bounded_iter<const_pointer, 0>;
@@ -320,7 +321,7 @@ struct _LIBCPP_TEMPLATE_VIS array<_Tp, 0> {
   using reverse_iterator       = std::reverse_iterator<iterator>;
   using const_reverse_iterator = std::reverse_iterator<const_iterator>;
 
-  using _EmptyType = __conditional_t<is_const<_Tp>::value, const __empty, __empty>;
+  using _EmptyType _LIBCPP_NODEBUG = __conditional_t<is_const<_Tp>::value, const __empty, __empty>;
 
   struct _ArrayInStructT {
     _Tp __data_[1];
diff --git a/libcxx/include/barrier b/libcxx/include/barrier
index 6861532b02ba..91dfa9720a37 100644
--- a/libcxx/include/barrier
+++ b/libcxx/include/barrier
@@ -95,7 +95,7 @@ It looks different from literature pseudocode for two main reasons:
 
 */
 
-using __barrier_phase_t = uint8_t;
+using __barrier_phase_t _LIBCPP_NODEBUG = uint8_t;
 
 class __barrier_algorithm_base;
 
diff --git a/libcxx/include/ccomplex b/libcxx/include/ccomplex
index 10eb8a36e417..ee7e088aac54 100644
--- a/libcxx/include/ccomplex
+++ b/libcxx/include/ccomplex
@@ -28,13 +28,14 @@
 
 #  if _LIBCPP_STD_VER >= 20
 
-using __standard_header_ccomplex _LIBCPP_DEPRECATED_("removed in C++20. Include <complex> instead.") = void;
-using __use_standard_header_ccomplex = __standard_header_ccomplex;
+using __standard_header_ccomplex
+    _LIBCPP_DEPRECATED_("removed in C++20. Include <complex> instead.") _LIBCPP_NODEBUG = void;
+using __use_standard_header_ccomplex _LIBCPP_NODEBUG                                    = __standard_header_ccomplex;
 
 #  elif _LIBCPP_STD_VER >= 17
 
-using __standard_header_ccomplex _LIBCPP_DEPRECATED_("Include <complex> instead.") = void;
-using __use_standard_header_ccomplex                                               = __standard_header_ccomplex;
+using __standard_header_ccomplex _LIBCPP_DEPRECATED_("Include <complex> instead.") _LIBCPP_NODEBUG = void;
+using __use_standard_header_ccomplex _LIBCPP_NODEBUG = __standard_header_ccomplex;
 
 #  endif
 #endif // __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
diff --git a/libcxx/include/ciso646 b/libcxx/include/ciso646
index 5b956401430a..34164362dc10 100644
--- a/libcxx/include/ciso646
+++ b/libcxx/include/ciso646
@@ -26,8 +26,9 @@
 
 #  if _LIBCPP_STD_VER >= 20
 
-using __standard_header_ciso646 _LIBCPP_DEPRECATED_("removed in C++20. Include <version> instead.") = void;
-using __use_standard_header_ciso646 = __standard_header_ciso646;
+using __standard_header_ciso646
+    _LIBCPP_DEPRECATED_("removed in C++20. Include <version> instead.") _LIBCPP_NODEBUG = void;
+using __use_standard_header_ciso646 _LIBCPP_NODEBUG                                     = __standard_header_ciso646;
 
 #  endif
 #endif // __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
diff --git a/libcxx/include/complex b/libcxx/include/complex
index bc73f313c2ce..df18159595b3 100644
--- a/libcxx/include/complex
+++ b/libcxx/include/complex
@@ -400,7 +400,7 @@ class _LIBCPP_TEMPLATE_VIS complex<long double>;
 struct __from_builtin_tag {};
 
 template <class _Tp>
-using __complex_t =
+using __complex_t _LIBCPP_NODEBUG =
     __conditional_t<is_same<_Tp, float>::value,
                     _Complex float,
                     __conditional_t<is_same<_Tp, double>::value, _Complex double, _Complex long double> >;
diff --git a/libcxx/include/cstdalign b/libcxx/include/cstdalign
index 6a277e467300..7f8dd1e1fbaf 100644
--- a/libcxx/include/cstdalign
+++ b/libcxx/include/cstdalign
@@ -45,13 +45,13 @@ Macros:
 
 #  if _LIBCPP_STD_VER >= 20
 
-using __standard_header_cstdalign _LIBCPP_DEPRECATED_("removed in C++20.") = void;
-using __use_standard_header_cstdalign                                      = __standard_header_cstdalign;
+using __standard_header_cstdalign _LIBCPP_DEPRECATED_("removed in C++20.") _LIBCPP_NODEBUG = void;
+using __use_standard_header_cstdalign _LIBCPP_NODEBUG = __standard_header_cstdalign;
 
 #  elif _LIBCPP_STD_VER >= 17
 
-using __standard_header_cstdalign _LIBCPP_DEPRECATED = void;
-using __use_standard_header_cstdalign                = __standard_header_cstdalign;
+using __standard_header_cstdalign _LIBCPP_DEPRECATED _LIBCPP_NODEBUG = void;
+using __use_standard_header_cstdalign _LIBCPP_NODEBUG                = __standard_header_cstdalign;
 
 #  endif
 #endif // __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
diff --git a/libcxx/include/cstdbool b/libcxx/include/cstdbool
index a12954f07398..a432d5f08b9a 100644
--- a/libcxx/include/cstdbool
+++ b/libcxx/include/cstdbool
@@ -33,13 +33,13 @@ Macros:
 
 #  if _LIBCPP_STD_VER >= 20
 
-using __standard_header_cstdbool _LIBCPP_DEPRECATED_("removed in C++20.") = void;
-using __use_standard_header_cstdbool                                      = __standard_header_cstdbool;
+using __standard_header_cstdbool _LIBCPP_DEPRECATED_("removed in C++20.") _LIBCPP_NODEBUG = void;
+using __use_standard_header_cstdbool _LIBCPP_NODEBUG                                      = __standard_header_cstdbool;
 
 #  elif _LIBCPP_STD_VER >= 17
 
-using __standard_header_cstdbool _LIBCPP_DEPRECATED = void;
-using __use_standard_header_cstdbool                = __standard_header_cstdbool;
+using __standard_header_cstdbool _LIBCPP_DEPRECATED _LIBCPP_NODEBUG = void;
+using __use_standard_header_cstdbool _LIBCPP_NODEBUG                = __standard_header_cstdbool;
 
 #  endif
 #endif // __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
diff --git a/libcxx/include/ctgmath b/libcxx/include/ctgmath
index 237f474f01c2..db0786f1e2c4 100644
--- a/libcxx/include/ctgmath
+++ b/libcxx/include/ctgmath
@@ -30,13 +30,14 @@
 
 #  if _LIBCPP_STD_VER >= 20
 
-using __standard_header_ctgmath _LIBCPP_DEPRECATED_("removed in C++20. Include <cmath> and <complex> instead.") = void;
-using __use_standard_header_ctgmath = __standard_header_ctgmath;
+using __standard_header_ctgmath
+    _LIBCPP_DEPRECATED_("removed in C++20. Include <cmath> and <complex> instead.") _LIBCPP_NODEBUG = void;
+using __use_standard_header_ctgmath _LIBCPP_NODEBUG = __standard_header_ctgmath;
 
 #  elif _LIBCPP_STD_VER >= 17
 
-using __standard_header_ctgmath _LIBCPP_DEPRECATED_("Include <cmath> and <complex> instead.") = void;
-using __use_standard_header_ctgmath = __standard_header_ctgmath;
+using __standard_header_ctgmath _LIBCPP_DEPRECATED_("Include <cmath> and <complex> instead.") _LIBCPP_NODEBUG = void;
+using __use_standard_header_ctgmath _LIBCPP_NODEBUG = __standard_header_ctgmath;
 
 #  endif
 
diff --git a/libcxx/include/deque b/libcxx/include/deque
index 883332f1ebdc..df3094cff7f8 100644
--- a/libcxx/include/deque
+++ b/libcxx/include/deque
@@ -456,12 +456,13 @@ template <class _ValueType, class _Pointer, class _Reference, class _MapPointer,
 struct __segmented_iterator_traits<
     __deque_iterator<_ValueType, _Pointer, _Reference, _MapPointer, _DiffType, _BlockSize> > {
 private:
-  using _Iterator = __deque_iterator<_ValueType, _Pointer, _Reference, _MapPointer, _DiffType, _BlockSize>;
+  using _Iterator _LIBCPP_NODEBUG =
+      __deque_iterator<_ValueType, _Pointer, _Reference, _MapPointer, _DiffType, _BlockSize>;
 
 public:
-  using __is_segmented_iterator = true_type;
-  using __segment_iterator      = _MapPointer;
-  using __local_iterator        = _Pointer;
+  using __is_segmented_iterator _LIBCPP_NODEBUG = true_type;
+  using __segment_iterator _LIBCPP_NODEBUG      = _MapPointer;
+  using __local_iterator _LIBCPP_NODEBUG        = _Pointer;
 
   static _LIBCPP_HIDE_FROM_ABI __segment_iterator __segment(_Iterator __iter) { return __iter.__m_iter_; }
   static _LIBCPP_HIDE_FROM_ABI __local_iterator __local(_Iterator __iter) { return __iter.__ptr_; }
@@ -491,8 +492,8 @@ public:
 
   using value_type = _Tp;
 
-  using allocator_type = _Allocator;
-  using __alloc_traits = allocator_traits<allocator_type>;
+  using allocator_type                 = _Allocator;
+  using __alloc_traits _LIBCPP_NODEBUG = allocator_traits<allocator_type>;
   static_assert(__check_valid_allocator<allocator_type>::value, "");
   static_assert(is_same<typename allocator_type::value_type, value_type>::value,
                 "Allocator::value_type must be same type as value_type");
@@ -503,13 +504,13 @@ public:
   using pointer       = typename __alloc_traits::pointer;
   using const_pointer = typename __alloc_traits::const_pointer;
 
-  using __pointer_allocator       = __rebind_alloc<__alloc_traits, pointer>;
-  using __const_pointer_allocator = __rebind_alloc<__alloc_traits, const_pointer>;
-  using __map                     = __split_buffer<pointer, __pointer_allocator>;
-  using __map_alloc_traits        = allocator_traits<__pointer_allocator>;
-  using __map_pointer             = typename __map_alloc_traits::pointer;
-  using __map_const_pointer       = typename allocator_traits<__const_pointer_allocator>::const_pointer;
-  using __map_const_iterator      = typename __map::const_iterator;
+  using __pointer_allocator _LIBCPP_NODEBUG       = __rebind_alloc<__alloc_traits, pointer>;
+  using __const_pointer_allocator _LIBCPP_NODEBUG = __rebind_alloc<__alloc_traits, const_pointer>;
+  using __map _LIBCPP_NODEBUG                     = __split_buffer<pointer, __pointer_allocator>;
+  using __map_alloc_traits _LIBCPP_NODEBUG        = allocator_traits<__pointer_allocator>;
+  using __map_pointer _LIBCPP_NODEBUG             = typename __map_alloc_traits::pointer;
+  using __map_const_pointer _LIBCPP_NODEBUG       = typename allocator_traits<__const_pointer_allocator>::const_pointer;
+  using __map_const_iterator _LIBCPP_NODEBUG      = typename __map::const_iterator;
 
   using reference       = value_type&;
   using const_reference = const value_type&;
@@ -525,7 +526,7 @@ public:
   // - size_type: is always trivially relocatable, since it is required to be an integral type
   // - allocator_type: may not be trivially relocatable, so it's checked
   // None of these are referencing the `deque` itself, so if all of them are trivially relocatable, `deque` is too.
-  using __trivially_relocatable = __conditional_t<
+  using __trivially_relocatable _LIBCPP_NODEBUG = __conditional_t<
       __libcpp_is_trivially_relocatable<__map>::value && __libcpp_is_trivially_relocatable<allocator_type>::value,
       deque,
       void>;
diff --git a/libcxx/include/experimental/__simd/scalar.h b/libcxx/include/experimental/__simd/scalar.h
index da318d2f4650..20c8b02c6589 100644
--- a/libcxx/include/experimental/__simd/scalar.h
+++ b/libcxx/include/experimental/__simd/scalar.h
@@ -49,8 +49,8 @@ struct __mask_storage<_Tp, simd_abi::__scalar> : __simd_storage<bool, simd_abi::
 
 template <class _Tp>
 struct __simd_operations<_Tp, simd_abi::__scalar> {
-  using _SimdStorage = __simd_storage<_Tp, simd_abi::__scalar>;
-  using _MaskStorage = __mask_storage<_Tp, simd_abi::__scalar>;
+  using _SimdStorage _LIBCPP_NODEBUG = __simd_storage<_Tp, simd_abi::__scalar>;
+  using _MaskStorage _LIBCPP_NODEBUG = __mask_storage<_Tp, simd_abi::__scalar>;
 
   static _LIBCPP_HIDE_FROM_ABI _SimdStorage __broadcast(_Tp __v) noexcept { return {__v}; }
 
@@ -86,7 +86,7 @@ struct __simd_operations<_Tp, simd_abi::__scalar> {
 
 template <class _Tp>
 struct __mask_operations<_Tp, simd_abi::__scalar> {
-  using _MaskStorage = __mask_storage<_Tp, simd_abi::__scalar>;
+  using _MaskStorage _LIBCPP_NODEBUG = __mask_storage<_Tp, simd_abi::__scalar>;
 
   static _LIBCPP_HIDE_FROM_ABI _MaskStorage __broadcast(bool __v) noexcept { return {__v}; }
 
diff --git a/libcxx/include/experimental/__simd/simd.h b/libcxx/include/experimental/__simd/simd.h
index fd919e75e32f..2fd2b2644d0b 100644
--- a/libcxx/include/experimental/__simd/simd.h
+++ b/libcxx/include/experimental/__simd/simd.h
@@ -43,8 +43,8 @@ public:
 // TODO: implement simd class
 template <class _Tp, class _Abi>
 class simd : public __simd_int_operators<simd<_Tp, _Abi>, __simd_operations<_Tp, _Abi>, is_integral_v<_Tp>> {
-  using _Impl    = __simd_operations<_Tp, _Abi>;
-  using _Storage = typename _Impl::_SimdStorage;
+  using _Impl _LIBCPP_NODEBUG    = __simd_operations<_Tp, _Abi>;
+  using _Storage _LIBCPP_NODEBUG = typename _Impl::_SimdStorage;
 
   _Storage __s_;
 
diff --git a/libcxx/include/experimental/__simd/simd_mask.h b/libcxx/include/experimental/__simd/simd_mask.h
index 6b6f671bf3e6..a11766545b43 100644
--- a/libcxx/include/experimental/__simd/simd_mask.h
+++ b/libcxx/include/experimental/__simd/simd_mask.h
@@ -27,8 +27,8 @@ inline namespace parallelism_v2 {
 // TODO: implement simd_mask class
 template <class _Tp, class _Abi>
 class simd_mask {
-  using _Impl    = __mask_operations<_Tp, _Abi>;
-  using _Storage = typename _Impl::_MaskStorage;
+  using _Impl _LIBCPP_NODEBUG    = __mask_operations<_Tp, _Abi>;
+  using _Storage _LIBCPP_NODEBUG = typename _Impl::_MaskStorage;
 
   _Storage __s_;
 
diff --git a/libcxx/include/experimental/__simd/vec_ext.h b/libcxx/include/experimental/__simd/vec_ext.h
index abc7e9595be9..2a4b8c748f86 100644
--- a/libcxx/include/experimental/__simd/vec_ext.h
+++ b/libcxx/include/experimental/__simd/vec_ext.h
@@ -55,8 +55,8 @@ struct __mask_storage<_Tp, simd_abi::__vec_ext<_Np>>
 
 template <class _Tp, int _Np>
 struct __simd_operations<_Tp, simd_abi::__vec_ext<_Np>> {
-  using _SimdStorage = __simd_storage<_Tp, simd_abi::__vec_ext<_Np>>;
-  using _MaskStorage = __mask_storage<_Tp, simd_abi::__vec_ext<_Np>>;
+  using _SimdStorage _LIBCPP_NODEBUG = __simd_storage<_Tp, simd_abi::__vec_ext<_Np>>;
+  using _MaskStorage _LIBCPP_NODEBUG = __mask_storage<_Tp, simd_abi::__vec_ext<_Np>>;
 
   static _LIBCPP_HIDE_FROM_ABI _SimdStorage __broadcast(_Tp __v) noexcept {
     _SimdStorage __result;
@@ -101,7 +101,7 @@ struct __simd_operations<_Tp, simd_abi::__vec_ext<_Np>> {
 
 template <class _Tp, int _Np>
 struct __mask_operations<_Tp, simd_abi::__vec_ext<_Np>> {
-  using _MaskStorage = __mask_storage<_Tp, simd_abi::__vec_ext<_Np>>;
+  using _MaskStorage _LIBCPP_NODEBUG = __mask_storage<_Tp, simd_abi::__vec_ext<_Np>>;
 
   static _LIBCPP_HIDE_FROM_ABI _MaskStorage __broadcast(bool __v) noexcept {
     _MaskStorage __result;
diff --git a/libcxx/include/forward_list b/libcxx/include/forward_list
index c1ab155d5a13..f3b9617ab2e0 100644
--- a/libcxx/include/forward_list
+++ b/libcxx/include/forward_list
@@ -202,6 +202,7 @@ template <class T, class Allocator, class Predicate>
 #  include <__algorithm/lexicographical_compare.h>
 #  include <__algorithm/lexicographical_compare_three_way.h>
 #  include <__algorithm/min.h>
+#  include <__assert>
 #  include <__config>
 #  include <__cstddef/nullptr_t.h>
 #  include <__iterator/distance.h>
@@ -316,7 +317,8 @@ struct __forward_begin_node {
 };
 
 template <class _Tp, class _VoidPtr>
-using __begin_node_of = __forward_begin_node<__rebind_pointer_t<_VoidPtr, __forward_list_node<_Tp, _VoidPtr> > >;
+using __begin_node_of _LIBCPP_NODEBUG =
+    __forward_begin_node<__rebind_pointer_t<_VoidPtr, __forward_list_node<_Tp, _VoidPtr> > >;
 
 template <class _Tp, class _VoidPtr>
 struct __forward_list_node : public __begin_node_of<_Tp, _VoidPtr> {
@@ -766,8 +768,14 @@ public:
     return std::min<size_type>(__node_traits::max_size(this->__alloc_), numeric_limits<difference_type>::max());
   }
 
-  _LIBCPP_HIDE_FROM_ABI reference front() { return __base::__before_begin()->__next_->__get_value(); }
-  _LIBCPP_HIDE_FROM_ABI const_reference front() const { return __base::__before_begin()->__next_->__get_value(); }
+  _LIBCPP_HIDE_FROM_ABI reference front() {
+    _LIBCPP_ASSERT_NON_NULL(!empty(), "forward_list::front called on an empty list");
+    return __base::__before_begin()->__next_->__get_value();
+  }
+  _LIBCPP_HIDE_FROM_ABI const_reference front() const {
+    _LIBCPP_ASSERT_NON_NULL(!empty(), "forward_list::front called on an empty list");
+    return __base::__before_begin()->__next_->__get_value();
+  }
 
 #  ifndef _LIBCPP_CXX03_LANG
 #    if _LIBCPP_STD_VER >= 17
@@ -1085,6 +1093,7 @@ void forward_list<_Tp, _Alloc>::push_front(const value_type& __v) {
 
 template <class _Tp, class _Alloc>
 void forward_list<_Tp, _Alloc>::pop_front() {
+  _LIBCPP_ASSERT_NON_NULL(!empty(), "forward_list::pop_front called on an empty list");
   __node_pointer __p                = __base::__before_begin()->__next_;
   __base::__before_begin()->__next_ = __p->__next_;
   this->__delete_node(__p);
diff --git a/libcxx/include/ios b/libcxx/include/ios
index 7c2ee83d4624..98a088266539 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -629,9 +629,9 @@ private:
   basic_ostream<char_type, traits_type>* __tie_;
 
 #    if defined(_LIBCPP_ABI_IOS_ALLOW_ARBITRARY_FILL_VALUE)
-  using _FillType = _FillHelper<traits_type>;
+  using _FillType _LIBCPP_NODEBUG = _FillHelper<traits_type>;
 #    else
-  using _FillType = _SentinelValueFill<traits_type>;
+  using _FillType _LIBCPP_NODEBUG = _SentinelValueFill<traits_type>;
 #    endif
   mutable _FillType __fill_;
 };
diff --git a/libcxx/include/optional b/libcxx/include/optional
index 165e0f16cab9..c325140ee66f 100644
--- a/libcxx/include/optional
+++ b/libcxx/include/optional
@@ -353,8 +353,8 @@ struct __optional_destruct_base<_Tp, true> {
 
 template <class _Tp, bool = is_reference<_Tp>::value>
 struct __optional_storage_base : __optional_destruct_base<_Tp> {
-  using __base     = __optional_destruct_base<_Tp>;
-  using value_type = _Tp;
+  using __base _LIBCPP_NODEBUG = __optional_destruct_base<_Tp>;
+  using value_type             = _Tp;
   using __base::__base;
 
   _LIBCPP_HIDE_FROM_ABI constexpr bool has_value() const noexcept { return this->__engaged_; }
@@ -396,8 +396,8 @@ struct __optional_storage_base : __optional_destruct_base<_Tp> {
 // to ensure we can make the change in an ABI-compatible manner.
 template <class _Tp>
 struct __optional_storage_base<_Tp, true> {
-  using value_type = _Tp;
-  using __raw_type = remove_reference_t<_Tp>;
+  using value_type                 = _Tp;
+  using __raw_type _LIBCPP_NODEBUG = remove_reference_t<_Tp>;
   __raw_type* __value_;
 
   template <class _Up>
@@ -555,11 +555,11 @@ struct __optional_move_assign_base<_Tp, false> : __optional_copy_assign_base<_Tp
 };
 
 template <class _Tp>
-using __optional_sfinae_ctor_base_t =
+using __optional_sfinae_ctor_base_t _LIBCPP_NODEBUG =
     __sfinae_ctor_base< is_copy_constructible<_Tp>::value, is_move_constructible<_Tp>::value >;
 
 template <class _Tp>
-using __optional_sfinae_assign_base_t =
+using __optional_sfinae_assign_base_t _LIBCPP_NODEBUG =
     __sfinae_assign_base< (is_copy_constructible<_Tp>::value && is_copy_assignable<_Tp>::value),
                           (is_move_constructible<_Tp>::value && is_move_assignable<_Tp>::value) >;
 
@@ -583,12 +583,13 @@ class _LIBCPP_DECLSPEC_EMPTY_BASES optional
     : private __optional_move_assign_base<_Tp>,
       private __optional_sfinae_ctor_base_t<_Tp>,
       private __optional_sfinae_assign_base_t<_Tp> {
-  using __base = __optional_move_assign_base<_Tp>;
+  using __base _LIBCPP_NODEBUG = __optional_move_assign_base<_Tp>;
 
 public:
   using value_type = _Tp;
 
-  using __trivially_relocatable = conditional_t<__libcpp_is_trivially_relocatable<_Tp>::value, optional, void>;
+  using __trivially_relocatable _LIBCPP_NODEBUG =
+      conditional_t<__libcpp_is_trivially_relocatable<_Tp>::value, optional, void>;
 
 private:
   // Disable the reference extension using this static assert.
@@ -613,7 +614,7 @@ private:
     }
   };
   template <class _Up>
-  using _CheckOptionalArgsCtor =
+  using _CheckOptionalArgsCtor _LIBCPP_NODEBUG =
       _If< _IsNotSame<__remove_cvref_t<_Up>, in_place_t>::value && _IsNotSame<__remove_cvref_t<_Up>, optional>::value &&
                (!is_same_v<remove_cv_t<_Tp>, bool> || !__is_std_optional<__remove_cvref_t<_Up>>::value),
            _CheckOptionalArgsConstructor,
@@ -621,7 +622,7 @@ private:
   template <class _QualUp>
   struct _CheckOptionalLikeConstructor {
     template <class _Up, class _Opt = optional<_Up>>
-    using __check_constructible_from_opt =
+    using __check_constructible_from_opt _LIBCPP_NODEBUG =
         _Or< is_constructible<_Tp, _Opt&>,
              is_constructible<_Tp, _Opt const&>,
              is_constructible<_Tp, _Opt&&>,
@@ -631,7 +632,7 @@ private:
              is_convertible<_Opt&&, _Tp>,
              is_convertible<_Opt const&&, _Tp> >;
     template <class _Up, class _Opt = optional<_Up>>
-    using __check_assignable_from_opt =
+    using __check_assignable_from_opt _LIBCPP_NODEBUG =
         _Or< is_assignable<_Tp&, _Opt&>,
              is_assignable<_Tp&, _Opt const&>,
              is_assignable<_Tp&, _Opt&&>,
@@ -655,12 +656,12 @@ private:
   };
 
   template <class _Up, class _QualUp>
-  using _CheckOptionalLikeCtor =
+  using _CheckOptionalLikeCtor _LIBCPP_NODEBUG =
       _If< _And< _IsNotSame<_Up, _Tp>, is_constructible<_Tp, _QualUp> >::value,
            _CheckOptionalLikeConstructor<_QualUp>,
            __check_tuple_constructor_fail >;
   template <class _Up, class _QualUp>
-  using _CheckOptionalLikeAssign =
+  using _CheckOptionalLikeAssign _LIBCPP_NODEBUG =
       _If< _And< _IsNotSame<_Up, _Tp>, is_constructible<_Tp, _QualUp>, is_assignable<_Tp&, _QualUp> >::value,
            _CheckOptionalLikeConstructor<_QualUp>,
            __check_tuple_constructor_fail >;
diff --git a/libcxx/include/ratio b/libcxx/include/ratio
index b35e2bd9dad6..2b5e34cbcd18 100644
--- a/libcxx/include/ratio
+++ b/libcxx/include/ratio
@@ -465,7 +465,7 @@ struct _LIBCPP_TEMPLATE_VIS ratio_greater_equal : _BoolConstant<!ratio_less<_R1,
 };
 
 template <class _R1, class _R2>
-using __ratio_gcd = ratio<__static_gcd<_R1::num, _R2::num>, __static_lcm<_R1::den, _R2::den> >;
+using __ratio_gcd _LIBCPP_NODEBUG = ratio<__static_gcd<_R1::num, _R2::num>, __static_lcm<_R1::den, _R2::den> >;
 
 #  if _LIBCPP_STD_VER >= 17
 template <class _R1, class _R2>
diff --git a/libcxx/include/regex b/libcxx/include/regex
index 15ec15a6985e..5cad0bc4b812 100644
--- a/libcxx/include/regex
+++ b/libcxx/include/regex
@@ -4229,7 +4229,8 @@ inline _LIBCPP_HIDE_FROM_ABI bool operator==(const sub_match<_BiIter>& __x, cons
 
 #  if _LIBCPP_STD_VER >= 20
 template <class _BiIter>
-using __sub_match_cat = compare_three_way_result_t<basic_string<typename iterator_traits<_BiIter>::value_type>>;
+using __sub_match_cat _LIBCPP_NODEBUG =
+    compare_three_way_result_t<basic_string<typename iterator_traits<_BiIter>::value_type>>;
 
 template <class _BiIter>
 _LIBCPP_HIDE_FROM_ABI auto operator<=>(const sub_match<_BiIter>& __x, const sub_match<_BiIter>& __y) {
diff --git a/libcxx/include/source_location b/libcxx/include/source_location
index bbbb86bc68c8..b4777ce5a100 100644
--- a/libcxx/include/source_location
+++ b/libcxx/include/source_location
@@ -55,7 +55,7 @@ class source_location {
   // in constant evaluation, so we don't want to use `void*` as the argument
   // type unless the builtin returned that, anyhow, and the invalid cast is
   // unavoidable.
-  using __bsl_ty = decltype(__builtin_source_location());
+  using __bsl_ty _LIBCPP_NODEBUG = decltype(__builtin_source_location());
 
 public:
   // The defaulted __ptr argument is necessary so that the builtin is evaluated
diff --git a/libcxx/include/string b/libcxx/include/string
index 7808f56f6001..39982d5670bd 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -764,7 +764,7 @@ struct __padding<0> {};
 template <class _CharT, class _Traits, class _Allocator>
 class basic_string {
 private:
-  using __default_allocator_type = allocator<_CharT>;
+  using __default_allocator_type _LIBCPP_NODEBUG = allocator<_CharT>;
 
 public:
   typedef basic_string __self;
@@ -798,7 +798,7 @@ public:
   // Therefore it's crucial to ensure the destructor is called.
   using __trivially_relocatable = void;
 #  else
-  using __trivially_relocatable = __conditional_t<
+  using __trivially_relocatable _LIBCPP_NODEBUG = __conditional_t<
       __libcpp_is_trivially_relocatable<allocator_type>::value && __libcpp_is_trivially_relocatable<pointer>::value,
       basic_string,
       void>;
diff --git a/libcxx/include/tuple b/libcxx/include/tuple
index b2478746f5e2..aca14ba408d3 100644
--- a/libcxx/include/tuple
+++ b/libcxx/include/tuple
@@ -552,7 +552,8 @@ class _LIBCPP_TEMPLATE_VIS tuple {
   get(const tuple<_Up...>&&) _NOEXCEPT;
 
 public:
-  using __trivially_relocatable = __conditional_t<_And<__libcpp_is_trivially_relocatable<_Tp>...>::value, tuple, void>;
+  using __trivially_relocatable _LIBCPP_NODEBUG =
+      __conditional_t<_And<__libcpp_is_trivially_relocatable<_Tp>...>::value, tuple, void>;
 
   // [tuple.cnstr]
 
@@ -1125,9 +1126,9 @@ inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 tuple<_Tp&...> tie(_T
 }
 
 template <class... _Tp>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 tuple<typename __unwrap_ref_decay<_Tp>::type...>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 tuple<__unwrap_ref_decay_t<_Tp>...>
 make_tuple(_Tp&&... __t) {
-  return tuple<typename __unwrap_ref_decay<_Tp>::type...>(std::forward<_Tp>(__t)...);
+  return tuple<__unwrap_ref_decay_t<_Tp>...>(std::forward<_Tp>(__t)...);
 }
 
 template <class... _Tp>
diff --git a/libcxx/include/variant b/libcxx/include/variant
index 3fa1b4b30f05..6c7be7f8f1eb 100644
--- a/libcxx/include/variant
+++ b/libcxx/include/variant
@@ -357,7 +357,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr auto __choose_index_type() {
 }
 
 template <size_t _NumAlts>
-using __variant_index_t = decltype(std::__choose_index_type<_NumAlts>());
+using __variant_index_t _LIBCPP_NODEBUG = decltype(std::__choose_index_type<_NumAlts>());
 
 template <class _IndexType>
 constexpr _IndexType __variant_npos = static_cast<_IndexType>(-1);
@@ -658,8 +658,8 @@ private:
 
 template <size_t _Index, class _Tp>
 struct _LIBCPP_TEMPLATE_VIS __alt {
-  using __value_type              = _Tp;
-  static constexpr size_t __index = _Index;
+  using __value_type _LIBCPP_NODEBUG = _Tp;
+  static constexpr size_t __index    = _Index;
 
   template <class... _Args>
   _LIBCPP_HIDE_FROM_ABI explicit constexpr __alt(in_place_t, _Args&&... __args)
@@ -713,7 +713,7 @@ _LIBCPP_VARIANT_UNION(_Trait::_Unavailable, _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTE
 template <_Trait _DestructibleTrait, class... _Types>
 class _LIBCPP_TEMPLATE_VIS __base {
 public:
-  using __index_t = __variant_index_t<sizeof...(_Types)>;
+  using __index_t _LIBCPP_NODEBUG = __variant_index_t<sizeof...(_Types)>;
 
   _LIBCPP_HIDE_FROM_ABI explicit constexpr __base(__valueless_t __tag) noexcept
       : __data(__tag), __index(__variant_npos<__index_t>) {}
@@ -753,8 +753,8 @@ class _LIBCPP_TEMPLATE_VIS __dtor;
       template <class... _Types>                                                                                       \
       class _LIBCPP_TEMPLATE_VIS __dtor<__traits<_Types...>, destructible_trait>                                       \
           : public __base<destructible_trait, _Types...> {                                                             \
-        using __base_type = __base<destructible_trait, _Types...>;                                                     \
-        using __index_t   = typename __base_type::__index_t;                                                           \
+        using __base_type _LIBCPP_NODEBUG = __base<destructible_trait, _Types...>;                                     \
+        using __index_t _LIBCPP_NODEBUG   = typename __base_type::__index_t;                                           \
                                                                                                                        \
       public:                                                                                                          \
         using __base_type::__base_type;                                                                                \
@@ -799,7 +799,7 @@ _LIBCPP_VARIANT_DESTRUCTOR(_Trait::_Unavailable,
 
 template <class _Traits>
 class _LIBCPP_TEMPLATE_VIS __ctor : public __dtor<_Traits> {
-  using __base_type = __dtor<_Traits>;
+  using __base_type _LIBCPP_NODEBUG = __dtor<_Traits>;
 
 public:
   using __base_type::__base_type;
@@ -831,7 +831,7 @@ class _LIBCPP_TEMPLATE_VIS __move_constructor;
       template <class... _Types>                                                                                       \
       class _LIBCPP_TEMPLATE_VIS __move_constructor<__traits<_Types...>, move_constructible_trait>                     \
           : public __ctor<__traits<_Types...>> {                                                                       \
-        using __base_type = __ctor<__traits<_Types...>>;                                                               \
+        using __base_type _LIBCPP_NODEBUG = __ctor<__traits<_Types...>>;                                               \
                                                                                                                        \
       public:                                                                                                          \
         using __base_type::__base_type;                                                                                \
@@ -869,7 +869,7 @@ class _LIBCPP_TEMPLATE_VIS __copy_constructor;
       template <class... _Types>                                                                                       \
       class _LIBCPP_TEMPLATE_VIS __copy_constructor<__traits<_Types...>, copy_constructible_trait>                     \
           : public __move_constructor<__traits<_Types...>> {                                                           \
-        using __base_type = __move_constructor<__traits<_Types...>>;                                                   \
+        using __base_type _LIBCPP_NODEBUG = __move_constructor<__traits<_Types...>>;                                   \
                                                                                                                        \
       public:                                                                                                          \
         using __base_type::__base_type;                                                                                \
@@ -899,7 +899,7 @@ _LIBCPP_VARIANT_COPY_CONSTRUCTOR(
 
 template <class _Traits>
 class _LIBCPP_TEMPLATE_VIS __assignment : public __copy_constructor<_Traits> {
-  using __base_type = __copy_constructor<_Traits>;
+  using __base_type _LIBCPP_NODEBUG = __copy_constructor<_Traits>;
 
 public:
   using __base_type::__base_type;
@@ -958,7 +958,7 @@ class _LIBCPP_TEMPLATE_VIS __move_assignment;
       template <class... _Types>                                                                                       \
       class _LIBCPP_TEMPLATE_VIS __move_assignment<__traits<_Types...>, move_assignable_trait>                         \
           : public __assignment<__traits<_Types...>> {                                                                 \
-        using __base_type = __assignment<__traits<_Types...>>;                                                         \
+        using __base_type _LIBCPP_NODEBUG = __assignment<__traits<_Types...>>;                                         \
                                                                                                                        \
       public:                                                                                                          \
         using __base_type::__base_type;                                                                                \
@@ -997,7 +997,7 @@ class _LIBCPP_TEMPLATE_VIS __copy_assignment;
       template <class... _Types>                                                                                       \
       class _LIBCPP_TEMPLATE_VIS __copy_assignment<__traits<_Types...>, copy_assignable_trait>                         \
           : public __move_assignment<__traits<_Types...>> {                                                            \
-        using __base_type = __move_assignment<__traits<_Types...>>;                                                    \
+        using __base_type _LIBCPP_NODEBUG = __move_assignment<__traits<_Types...>>;                                    \
                                                                                                                        \
       public:                                                                                                          \
         using __base_type::__base_type;                                                                                \
@@ -1030,7 +1030,7 @@ _LIBCPP_VARIANT_COPY_ASSIGNMENT(_Trait::_Unavailable,
 
 template <class... _Types>
 class _LIBCPP_TEMPLATE_VIS __impl : public __copy_assignment<__traits<_Types...>> {
-  using __base_type = __copy_assignment<__traits<_Types...>>;
+  using __base_type _LIBCPP_NODEBUG = __copy_assignment<__traits<_Types...>>;
 
 public:
   using __base_type::__base_type; // get in_place_index_t constructor & friends
@@ -1097,7 +1097,7 @@ private:
 
 struct __no_narrowing_check {
   template <class _Dest, class _Source>
-  using _Apply = __type_identity<_Dest>;
+  using _Apply _LIBCPP_NODEBUG = __type_identity<_Dest>;
 };
 
 struct __narrowing_check {
@@ -1138,7 +1138,7 @@ using _MakeOverloads _LIBCPP_NODEBUG =
     typename __make_overloads_imp< __make_indices_imp<sizeof...(_Types), 0> >::template _Apply<_Types...>;
 
 template <class _Tp, class... _Types>
-using __best_match_t = typename invoke_result_t<_MakeOverloads<_Types...>, _Tp, _Tp>::type;
+using __best_match_t _LIBCPP_NODEBUG = typename invoke_result_t<_MakeOverloads<_Types...>, _Tp, _Tp>::type;
 
 } // namespace __variant_detail
 
@@ -1170,10 +1170,10 @@ class _LIBCPP_TEMPLATE_VIS _LIBCPP_DECLSPEC_EMPTY_BASES variant
 
   static_assert(__all<!is_void_v<_Types>...>::value, "variant can not have a void type as an alternative.");
 
-  using __first_type = variant_alternative_t<0, variant>;
+  using __first_type _LIBCPP_NODEBUG = variant_alternative_t<0, variant>;
 
 public:
-  using __trivially_relocatable =
+  using __trivially_relocatable _LIBCPP_NODEBUG =
       conditional_t<_And<__libcpp_is_trivially_relocatable<_Types>...>::value, variant, void>;
 
   template <bool _Dummy                                                                               = true,
diff --git a/libcxx/src/experimental/tzdb.cpp b/libcxx/src/experimental/tzdb.cpp
index d22de21c9981..638d45f69e03 100644
--- a/libcxx/src/experimental/tzdb.cpp
+++ b/libcxx/src/experimental/tzdb.cpp
@@ -9,11 +9,14 @@
 // For information see https://libcxx.llvm.org/DesignDocs/TimeZone.html
 
 #include <algorithm>
+#include <cctype>
 #include <chrono>
 #include <filesystem>
 #include <fstream>
 #include <stdexcept>
 #include <string>
+#include <string_view>
+#include <vector>
 
 #include "include/tzdb/time_zone_private.h"
 #include "include/tzdb/types_private.h"
diff --git a/libcxx/src/filesystem/directory_iterator.cpp b/libcxx/src/filesystem/directory_iterator.cpp
index d7ed9a358f55..7e8e40d17f7a 100644
--- a/libcxx/src/filesystem/directory_iterator.cpp
+++ b/libcxx/src/filesystem/directory_iterator.cpp
@@ -47,9 +47,9 @@ public:
     }
     __stream_ = ::FindFirstFileW((root / "*").c_str(), &__data_);
     if (__stream_ == INVALID_HANDLE_VALUE) {
-      ec                                  = detail::make_windows_error(GetLastError());
+      ec                                  = detail::get_last_error();
       const bool ignore_permission_denied = bool(opts & directory_options::skip_permission_denied);
-      if (ignore_permission_denied && ec.value() == static_cast<int>(errc::permission_denied))
+      if (ignore_permission_denied && ec == errc::permission_denied)
         ec.clear();
       return;
     }
@@ -91,7 +91,7 @@ private:
   error_code close() noexcept {
     error_code ec;
     if (!::FindClose(__stream_))
-      ec = detail::make_windows_error(GetLastError());
+      ec = detail::get_last_error();
     __stream_ = INVALID_HANDLE_VALUE;
     return ec;
   }
@@ -118,7 +118,7 @@ public:
     if ((__stream_ = ::opendir(root.c_str())) == nullptr) {
       ec                      = detail::capture_errno();
       const bool allow_eacces = bool(opts & directory_options::skip_permission_denied);
-      if (allow_eacces && ec.value() == EACCES)
+      if (allow_eacces && ec == errc::permission_denied)
         ec.clear();
       return;
     }
@@ -307,7 +307,7 @@ bool recursive_directory_iterator::__try_recursion(error_code* ec) {
   }
   if (m_ec) {
     const bool allow_eacess = bool(__imp_->__options_ & directory_options::skip_permission_denied);
-    if (m_ec.value() == EACCES && allow_eacess) {
+    if (m_ec == errc::permission_denied && allow_eacess) {
       if (ec)
         ec->clear();
     } else {
diff --git a/libcxx/src/filesystem/error.h b/libcxx/src/filesystem/error.h
index 07ba7fc3eef2..c0213910b378 100644
--- a/libcxx/src/filesystem/error.h
+++ b/libcxx/src/filesystem/error.h
@@ -32,80 +32,21 @@ _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM
 
 namespace detail {
 
-#if defined(_LIBCPP_WIN32API)
-
-inline errc __win_err_to_errc(int err) {
-  constexpr struct {
-    DWORD win;
-    errc errc;
-  } win_error_mapping[] = {
-      {ERROR_ACCESS_DENIED, errc::permission_denied},
-      {ERROR_ALREADY_EXISTS, errc::file_exists},
-      {ERROR_BAD_NETPATH, errc::no_such_file_or_directory},
-      {ERROR_BAD_PATHNAME, errc::no_such_file_or_directory},
-      {ERROR_BAD_UNIT, errc::no_such_device},
-      {ERROR_BROKEN_PIPE, errc::broken_pipe},
-      {ERROR_BUFFER_OVERFLOW, errc::filename_too_long},
-      {ERROR_BUSY, errc::device_or_resource_busy},
-      {ERROR_BUSY_DRIVE, errc::device_or_resource_busy},
-      {ERROR_CANNOT_MAKE, errc::permission_denied},
-      {ERROR_CANTOPEN, errc::io_error},
-      {ERROR_CANTREAD, errc::io_error},
-      {ERROR_CANTWRITE, errc::io_error},
-      {ERROR_CURRENT_DIRECTORY, errc::permission_denied},
-      {ERROR_DEV_NOT_EXIST, errc::no_such_device},
-      {ERROR_DEVICE_IN_USE, errc::device_or_resource_busy},
-      {ERROR_DIR_NOT_EMPTY, errc::directory_not_empty},
-      {ERROR_DIRECTORY, errc::invalid_argument},
-      {ERROR_DISK_FULL, errc::no_space_on_device},
-      {ERROR_FILE_EXISTS, errc::file_exists},
-      {ERROR_FILE_NOT_FOUND, errc::no_such_file_or_directory},
-      {ERROR_HANDLE_DISK_FULL, errc::no_space_on_device},
-      {ERROR_INVALID_ACCESS, errc::permission_denied},
-      {ERROR_INVALID_DRIVE, errc::no_such_device},
-      {ERROR_INVALID_FUNCTION, errc::function_not_supported},
-      {ERROR_INVALID_HANDLE, errc::invalid_argument},
-      {ERROR_INVALID_NAME, errc::no_such_file_or_directory},
-      {ERROR_INVALID_PARAMETER, errc::invalid_argument},
-      {ERROR_LOCK_VIOLATION, errc::no_lock_available},
-      {ERROR_LOCKED, errc::no_lock_available},
-      {ERROR_NEGATIVE_SEEK, errc::invalid_argument},
-      {ERROR_NOACCESS, errc::permission_denied},
-      {ERROR_NOT_ENOUGH_MEMORY, errc::not_enough_memory},
-      {ERROR_NOT_READY, errc::resource_unavailable_try_again},
-      {ERROR_NOT_SAME_DEVICE, errc::cross_device_link},
-      {ERROR_NOT_SUPPORTED, errc::not_supported},
-      {ERROR_OPEN_FAILED, errc::io_error},
-      {ERROR_OPEN_FILES, errc::device_or_resource_busy},
-      {ERROR_OPERATION_ABORTED, errc::operation_canceled},
-      {ERROR_OUTOFMEMORY, errc::not_enough_memory},
-      {ERROR_PATH_NOT_FOUND, errc::no_such_file_or_directory},
-      {ERROR_READ_FAULT, errc::io_error},
-      {ERROR_REPARSE_TAG_INVALID, errc::invalid_argument},
-      {ERROR_RETRY, errc::resource_unavailable_try_again},
-      {ERROR_SEEK, errc::io_error},
-      {ERROR_SHARING_VIOLATION, errc::permission_denied},
-      {ERROR_TOO_MANY_OPEN_FILES, errc::too_many_files_open},
-      {ERROR_WRITE_FAULT, errc::io_error},
-      {ERROR_WRITE_PROTECT, errc::permission_denied},
-  };
-
-  for (const auto& pair : win_error_mapping)
-    if (pair.win == static_cast<DWORD>(err))
-      return pair.errc;
-  return errc::invalid_argument;
-}
-
-#endif // _LIBCPP_WIN32API
+// On windows, libc functions use errno, but system functions use GetLastError.
+// So, callers need to be careful which of these next functions they call!
 
 inline error_code capture_errno() {
   _LIBCPP_ASSERT_INTERNAL(errno != 0, "Expected errno to be non-zero");
   return error_code(errno, generic_category());
 }
 
+inline error_code get_last_error() {
 #if defined(_LIBCPP_WIN32API)
-inline error_code make_windows_error(int err) { return make_error_code(__win_err_to_errc(err)); }
+  return std::error_code(GetLastError(), std::system_category());
+#else
+  return capture_errno();
 #endif
+}
 
 template <class T>
 T error_value();
diff --git a/libcxx/src/filesystem/file_descriptor.h b/libcxx/src/filesystem/file_descriptor.h
index db66ad55bd4f..9c279c451f28 100644
--- a/libcxx/src/filesystem/file_descriptor.h
+++ b/libcxx/src/filesystem/file_descriptor.h
@@ -201,7 +201,7 @@ inline perms posix_get_perms(const StatT& st) noexcept { return static_cast<perm
 inline file_status create_file_status(error_code& m_ec, path const& p, const StatT& path_stat, error_code* ec) {
   if (ec)
     *ec = m_ec;
-  if (m_ec && (m_ec.value() == ENOENT || m_ec.value() == ENOTDIR)) {
+  if (m_ec && (m_ec == errc::no_such_file_or_directory || m_ec == errc::not_a_directory)) {
     return file_status(file_type::not_found);
   } else if (m_ec) {
     ErrorHandler<void> err("posix_stat", ec, &p);
@@ -236,7 +236,7 @@ inline file_status create_file_status(error_code& m_ec, path const& p, const Sta
 inline file_status posix_stat(path const& p, StatT& path_stat, error_code* ec) {
   error_code m_ec;
   if (detail::stat(p.c_str(), &path_stat) == -1)
-    m_ec = detail::capture_errno();
+    m_ec = detail::get_last_error();
   return create_file_status(m_ec, p, path_stat, ec);
 }
 
@@ -248,7 +248,7 @@ inline file_status posix_stat(path const& p, error_code* ec) {
 inline file_status posix_lstat(path const& p, StatT& path_stat, error_code* ec) {
   error_code m_ec;
   if (detail::lstat(p.c_str(), &path_stat) == -1)
-    m_ec = detail::capture_errno();
+    m_ec = detail::get_last_error();
   return create_file_status(m_ec, p, path_stat, ec);
 }
 
@@ -260,7 +260,7 @@ inline file_status posix_lstat(path const& p, error_code* ec) {
 // http://pubs.opengroup.org/onlinepubs/9699919799/functions/ftruncate.html
 inline bool posix_ftruncate(const FileDescriptor& fd, off_t to_size, error_code& ec) {
   if (detail::ftruncate(fd.fd, to_size) == -1) {
-    ec = capture_errno();
+    ec = get_last_error();
     return true;
   }
   ec.clear();
@@ -269,7 +269,7 @@ inline bool posix_ftruncate(const FileDescriptor& fd, off_t to_size, error_code&
 
 inline bool posix_fchmod(const FileDescriptor& fd, const StatT& st, error_code& ec) {
   if (detail::fchmod(fd.fd, st.st_mode) == -1) {
-    ec = capture_errno();
+    ec = get_last_error();
     return true;
   }
   ec.clear();
@@ -286,7 +286,7 @@ inline file_status FileDescriptor::refresh_status(error_code& ec) {
   m_stat   = {};
   error_code m_ec;
   if (detail::fstat(fd, &m_stat) == -1)
-    m_ec = capture_errno();
+    m_ec = get_last_error();
   m_status = create_file_status(m_ec, name, m_stat, &ec);
   return m_status;
 }
diff --git a/libcxx/src/filesystem/operations.cpp b/libcxx/src/filesystem/operations.cpp
index d771f2009735..23c1c281ba1c 100644
--- a/libcxx/src/filesystem/operations.cpp
+++ b/libcxx/src/filesystem/operations.cpp
@@ -15,6 +15,7 @@
 #include <filesystem>
 #include <iterator>
 #include <string_view>
+#include <system_error>
 #include <type_traits>
 #include <vector>
 
@@ -32,11 +33,24 @@
 #  include <dirent.h>
 #  include <sys/stat.h>
 #  include <sys/statvfs.h>
+#  include <sys/types.h>
 #  include <unistd.h>
 #endif
 #include <fcntl.h> /* values for fchmodat */
 #include <time.h>
 
+// since Linux 4.5 and FreeBSD 13, but the Linux libc wrapper is only provided by glibc >= 2.27 and musl
+#if defined(__linux__)
+#  if defined(_LIBCPP_GLIBC_PREREQ)
+#    if _LIBCPP_GLIBC_PREREQ(2, 27)
+#      define _LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE
+#    endif
+#  elif _LIBCPP_HAS_MUSL_LIBC
+#    define _LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE
+#  endif
+#elif defined(__FreeBSD__)
+#  define _LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE
+#endif
 #if __has_include(<sys/sendfile.h>)
 #  include <sys/sendfile.h>
 #  define _LIBCPP_FILESYSTEM_USE_SENDFILE
@@ -44,10 +58,18 @@
 #  include <copyfile.h>
 #  define _LIBCPP_FILESYSTEM_USE_COPYFILE
 #else
-#  include <fstream>
 #  define _LIBCPP_FILESYSTEM_USE_FSTREAM
 #endif
 
+// sendfile and copy_file_range need to fall back
+// to the fstream implementation for special files
+#if (defined(_LIBCPP_FILESYSTEM_USE_SENDFILE) || defined(_LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE) ||                    \
+     defined(_LIBCPP_FILESYSTEM_USE_FSTREAM)) &&                                                                       \
+    _LIBCPP_HAS_LOCALIZATION
+#  include <fstream>
+#  define _LIBCPP_FILESYSTEM_NEED_FSTREAM
+#endif
+
 #if defined(__ELF__) && defined(_LIBCPP_LINK_RT_LIB)
 #  pragma comment(lib, "rt")
 #endif
@@ -86,7 +108,7 @@ path __canonical(path const& orig_p, error_code* ec) {
 #if (defined(_POSIX_VERSION) && _POSIX_VERSION >= 200112) || defined(_LIBCPP_WIN32API)
   std::unique_ptr<path::value_type, decltype(&::free)> hold(detail::realpath(p.c_str(), nullptr), &::free);
   if (hold.get() == nullptr)
-    return err.report(capture_errno());
+    return err.report(detail::get_last_error());
   return {hold.get()};
 #else
 #  if defined(__MVS__) && !defined(PATH_MAX)
@@ -96,7 +118,7 @@ path __canonical(path const& orig_p, error_code* ec) {
 #  endif
   path::value_type* ret;
   if ((ret = detail::realpath(p.c_str(), buff)) == nullptr)
-    return err.report(capture_errno());
+    return err.report(detail::get_last_error());
   return {ret};
 #endif
 }
@@ -178,9 +200,89 @@ void __copy(const path& from, const path& to, copy_options options, error_code*
 namespace detail {
 namespace {
 
+#if defined(_LIBCPP_FILESYSTEM_NEED_FSTREAM)
+bool copy_file_impl_fstream(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
+  ifstream in;
+  in.__open(read_fd.fd, ios::binary);
+  if (!in.is_open()) {
+    // This assumes that __open didn't reset the error code.
+    ec = capture_errno();
+    return false;
+  }
+  read_fd.fd = -1;
+  ofstream out;
+  out.__open(write_fd.fd, ios::binary);
+  if (!out.is_open()) {
+    ec = capture_errno();
+    return false;
+  }
+  write_fd.fd = -1;
+
+  if (in.good() && out.good()) {
+    using InIt  = istreambuf_iterator<char>;
+    using OutIt = ostreambuf_iterator<char>;
+    InIt bin(in);
+    InIt ein;
+    OutIt bout(out);
+    copy(bin, ein, bout);
+  }
+  if (out.fail() || in.fail()) {
+    ec = make_error_code(errc::io_error);
+    return false;
+  }
+
+  ec.clear();
+  return true;
+}
+#endif
+
+#if defined(_LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE)
+bool copy_file_impl_copy_file_range(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
+  size_t count = read_fd.get_stat().st_size;
+  // a zero-length file is either empty, or not copyable by this syscall
+  // return early to avoid the syscall cost
+  if (count == 0) {
+    ec = {EINVAL, generic_category()};
+    return false;
+  }
+  // do not modify the fd positions as copy_file_impl_sendfile may be called after a partial copy
+#  if defined(__linux__)
+  loff_t off_in  = 0;
+  loff_t off_out = 0;
+#  else
+  off_t off_in  = 0;
+  off_t off_out = 0;
+#  endif
+
+  do {
+    ssize_t res;
+
+    if ((res = ::copy_file_range(read_fd.fd, &off_in, write_fd.fd, &off_out, count, 0)) == -1) {
+      ec = capture_errno();
+      return false;
+    }
+    count -= res;
+  } while (count > 0);
+
+  ec.clear();
+
+  return true;
+}
+#endif
+
 #if defined(_LIBCPP_FILESYSTEM_USE_SENDFILE)
-bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
+bool copy_file_impl_sendfile(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
   size_t count = read_fd.get_stat().st_size;
+  // a zero-length file is either empty, or not copyable by this syscall
+  // return early to avoid the syscall cost
+  // however, we can't afford this luxury in the no-locale build,
+  // as we can't utilize the fstream impl to copy empty files
+#  if _LIBCPP_HAS_LOCALIZATION
+  if (count == 0) {
+    ec = {EINVAL, generic_category()};
+    return false;
+  }
+#  endif
   do {
     ssize_t res;
     if ((res = ::sendfile(write_fd.fd, read_fd.fd, nullptr, count)) == -1) {
@@ -194,6 +296,54 @@ bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_cod
 
   return true;
 }
+#endif
+
+#if defined(_LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE) || defined(_LIBCPP_FILESYSTEM_USE_SENDFILE)
+// If we have copy_file_range or sendfile, try both in succession (if available).
+// If both fail, fall back to using fstream.
+bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
+#  if defined(_LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE)
+  if (copy_file_impl_copy_file_range(read_fd, write_fd, ec)) {
+    return true;
+  }
+  // EINVAL: src and dst are the same file (this is not cheaply
+  // detectable from userspace)
+  // EINVAL: copy_file_range is unsupported for this file type by the
+  // underlying filesystem
+  // ENOTSUP: undocumented, can arise with old kernels and NFS
+  // EOPNOTSUPP: filesystem does not implement copy_file_range
+  // ETXTBSY: src or dst is an active swapfile (nonsensical, but allowed
+  // with normal copying)
+  // EXDEV: src and dst are on different filesystems that do not support
+  // cross-fs copy_file_range
+  // ENOENT: undocumented, can arise with CIFS
+  // ENOSYS: unsupported by kernel or blocked by seccomp
+  if (ec.value() != EINVAL && ec.value() != ENOTSUP && ec.value() != EOPNOTSUPP && ec.value() != ETXTBSY &&
+      ec.value() != EXDEV && ec.value() != ENOENT && ec.value() != ENOSYS) {
+    return false;
+  }
+  ec.clear();
+#  endif
+
+#  if defined(_LIBCPP_FILESYSTEM_USE_SENDFILE)
+  if (copy_file_impl_sendfile(read_fd, write_fd, ec)) {
+    return true;
+  }
+  // EINVAL: unsupported file type
+  if (ec.value() != EINVAL) {
+    return false;
+  }
+  ec.clear();
+#  endif
+
+#  if defined(_LIBCPP_FILESYSTEM_NEED_FSTREAM)
+  return copy_file_impl_fstream(read_fd, write_fd, ec);
+#  else
+  // since iostreams are unavailable in the no-locale build, just fail after a failed sendfile
+  ec.assign(EINVAL, std::system_category());
+  return false;
+#  endif
+}
 #elif defined(_LIBCPP_FILESYSTEM_USE_COPYFILE)
 bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
   struct CopyFileState {
@@ -217,37 +367,7 @@ bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_cod
 }
 #elif defined(_LIBCPP_FILESYSTEM_USE_FSTREAM)
 bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
-  ifstream in;
-  in.__open(read_fd.fd, ios::binary);
-  if (!in.is_open()) {
-    // This assumes that __open didn't reset the error code.
-    ec = capture_errno();
-    return false;
-  }
-  read_fd.fd = -1;
-  ofstream out;
-  out.__open(write_fd.fd, ios::binary);
-  if (!out.is_open()) {
-    ec = capture_errno();
-    return false;
-  }
-  write_fd.fd = -1;
-
-  if (in.good() && out.good()) {
-    using InIt  = istreambuf_iterator<char>;
-    using OutIt = ostreambuf_iterator<char>;
-    InIt bin(in);
-    InIt ein;
-    OutIt bout(out);
-    copy(bin, ein, bout);
-  }
-  if (out.fail() || in.fail()) {
-    ec = make_error_code(errc::io_error);
-    return false;
-  }
-
-  ec.clear();
-  return true;
+  return copy_file_impl_fstream(read_fd, write_fd, ec);
 }
 #else
 #  error "Unknown implementation for copy_file_impl"
@@ -393,9 +513,9 @@ bool __create_directory(const path& p, error_code* ec) {
   if (detail::mkdir(p.c_str(), static_cast<int>(perms::all)) == 0)
     return true;
 
-  if (errno != EEXIST)
-    return err.report(capture_errno());
-  error_code mec = capture_errno();
+  error_code mec = detail::get_last_error();
+  if (mec != errc::file_exists)
+    return err.report(mec);
   error_code ignored_ec;
   const file_status st = status(p, ignored_ec);
   if (!is_directory(st))
@@ -417,10 +537,10 @@ bool __create_directory(path const& p, path const& attributes, error_code* ec) {
   if (detail::mkdir(p.c_str(), attr_stat.st_mode) == 0)
     return true;
 
-  if (errno != EEXIST)
-    return err.report(capture_errno());
+  mec = detail::get_last_error();
+  if (mec != errc::file_exists)
+    return err.report(mec);
 
-  mec = capture_errno();
   error_code ignored_ec;
   st = status(p, ignored_ec);
   if (!is_directory(st))
@@ -431,19 +551,19 @@ bool __create_directory(path const& p, path const& attributes, error_code* ec) {
 void __create_directory_symlink(path const& from, path const& to, error_code* ec) {
   ErrorHandler<void> err("create_directory_symlink", ec, &from, &to);
   if (detail::symlink_dir(from.c_str(), to.c_str()) == -1)
-    return err.report(capture_errno());
+    return err.report(detail::get_last_error());
 }
 
 void __create_hard_link(const path& from, const path& to, error_code* ec) {
   ErrorHandler<void> err("create_hard_link", ec, &from, &to);
   if (detail::link(from.c_str(), to.c_str()) == -1)
-    return err.report(capture_errno());
+    return err.report(detail::get_last_error());
 }
 
 void __create_symlink(path const& from, path const& to, error_code* ec) {
   ErrorHandler<void> err("create_symlink", ec, &from, &to);
   if (detail::symlink_file(from.c_str(), to.c_str()) == -1)
-    return err.report(capture_errno());
+    return err.report(detail::get_last_error());
 }
 
 path __current_path(error_code* ec) {
@@ -486,7 +606,7 @@ path __current_path(error_code* ec) {
 
   unique_ptr<path::value_type, Deleter> hold(detail::getcwd(ptr, size), deleter);
   if (hold.get() == nullptr)
-    return err.report(capture_errno(), "call to getcwd failed");
+    return err.report(detail::get_last_error(), "call to getcwd failed");
 
   return {hold.get()};
 }
@@ -494,7 +614,7 @@ path __current_path(error_code* ec) {
 void __current_path(const path& p, error_code* ec) {
   ErrorHandler<void> err("current_path", ec, &p);
   if (detail::chdir(p.c_str()) == -1)
-    err.report(capture_errno());
+    err.report(detail::get_last_error());
 }
 
 bool __equivalent(const path& p1, const path& p2, error_code* ec) {
@@ -582,10 +702,10 @@ void __last_write_time(const path& p, file_time_type new_time, error_code* ec) {
     return err.report(errc::value_too_large);
   detail::WinHandle h(p.c_str(), FILE_WRITE_ATTRIBUTES, 0);
   if (!h)
-    return err.report(detail::make_windows_error(GetLastError()));
+    return err.report(detail::get_last_error());
   FILETIME last_write = timespec_to_filetime(ts);
   if (!SetFileTime(h, nullptr, nullptr, &last_write))
-    return err.report(detail::make_windows_error(GetLastError()));
+    return err.report(detail::get_last_error());
 #else
   error_code m_ec;
   array<TimeSpec, 2> tbuf;
@@ -643,7 +763,7 @@ void __permissions(const path& p, perms prms, perm_options opts, error_code* ec)
 #if defined(AT_SYMLINK_NOFOLLOW) && defined(AT_FDCWD)
   const int flags = set_sym_perms ? AT_SYMLINK_NOFOLLOW : 0;
   if (detail::fchmodat(AT_FDCWD, p.c_str(), real_perms, flags) == -1) {
-    return err.report(capture_errno());
+    return err.report(detail::get_last_error());
   }
 #else
   if (set_sym_perms)
@@ -671,14 +791,14 @@ path __read_symlink(const path& p, error_code* ec) {
 #else
   StatT sb;
   if (detail::lstat(p.c_str(), &sb) == -1) {
-    return err.report(capture_errno());
+    return err.report(detail::get_last_error());
   }
   const size_t size = sb.st_size + 1;
   auto buff         = unique_ptr<path::value_type[]>(new path::value_type[size]);
 #endif
   detail::SSizeT ret;
   if ((ret = detail::readlink(p.c_str(), buff.get(), size)) == -1)
-    return err.report(capture_errno());
+    return err.report(detail::get_last_error());
   // Note that `ret` returning `0` would work, resulting in a valid empty string being returned.
   if (static_cast<size_t>(ret) >= size)
     return err.report(errc::value_too_large);
@@ -689,8 +809,9 @@ path __read_symlink(const path& p, error_code* ec) {
 bool __remove(const path& p, error_code* ec) {
   ErrorHandler<bool> err("remove", ec, &p);
   if (detail::remove(p.c_str()) == -1) {
-    if (errno != ENOENT)
-      err.report(capture_errno());
+    error_code mec = detail::get_last_error();
+    if (mec != errc::no_such_file_or_directory)
+      err.report(mec);
     return false;
   }
   return true;
@@ -843,13 +964,13 @@ uintmax_t __remove_all(const path& p, error_code* ec) {
 void __rename(const path& from, const path& to, error_code* ec) {
   ErrorHandler<void> err("rename", ec, &from, &to);
   if (detail::rename(from.c_str(), to.c_str()) == -1)
-    err.report(capture_errno());
+    err.report(detail::get_last_error());
 }
 
 void __resize_file(const path& p, uintmax_t size, error_code* ec) {
   ErrorHandler<void> err("resize_file", ec, &p);
   if (detail::truncate(p.c_str(), static_cast< ::off_t>(size)) == -1)
-    return err.report(capture_errno());
+    return err.report(detail::get_last_error());
 }
 
 space_info __space(const path& p, error_code* ec) {
@@ -857,7 +978,7 @@ space_info __space(const path& p, error_code* ec) {
   space_info si;
   detail::StatVFS m_svfs = {};
   if (detail::statvfs(p.c_str(), &m_svfs) == -1) {
-    err.report(capture_errno());
+    err.report(detail::get_last_error());
     si.capacity = si.free = si.available = static_cast<uintmax_t>(-1);
     return si;
   }
@@ -884,7 +1005,7 @@ path __temp_directory_path(error_code* ec) {
   wchar_t buf[MAX_PATH];
   DWORD retval = GetTempPathW(MAX_PATH, buf);
   if (!retval)
-    return err.report(detail::make_windows_error(GetLastError()));
+    return err.report(detail::get_last_error());
   if (retval > MAX_PATH)
     return err.report(errc::filename_too_long);
   // GetTempPathW returns a path with a trailing slash, which we
diff --git a/libcxx/src/filesystem/posix_compat.h b/libcxx/src/filesystem/posix_compat.h
index b41c004341af..ddd99d8aaf20 100644
--- a/libcxx/src/filesystem/posix_compat.h
+++ b/libcxx/src/filesystem/posix_compat.h
@@ -11,9 +11,10 @@
 //
 // These generally behave like the proper posix functions, with these
 // exceptions:
-// On Windows, they take paths in wchar_t* form, instead of char* form.
-// The symlink() function is split into two frontends, symlink_file()
-// and symlink_dir().
+// - On Windows, they take paths in wchar_t* form, instead of char* form.
+// - The symlink() function is split into two frontends, symlink_file()
+//   and symlink_dir().
+// - Errors should be retrieved with get_last_error, not errno.
 //
 // These are provided within an anonymous namespace within the detail
 // namespace - callers need to include this header and call them as
@@ -122,11 +123,6 @@ namespace detail {
 
 #  define O_NONBLOCK 0
 
-inline int set_errno(int e = GetLastError()) {
-  errno = static_cast<int>(__win_err_to_errc(e));
-  return -1;
-}
-
 class WinHandle {
 public:
   WinHandle(const wchar_t* p, DWORD access, DWORD flags) {
@@ -153,7 +149,7 @@ private:
 inline int stat_handle(HANDLE h, StatT* buf) {
   FILE_BASIC_INFO basic;
   if (!GetFileInformationByHandleEx(h, FileBasicInfo, &basic, sizeof(basic)))
-    return set_errno();
+    return -1;
   memset(buf, 0, sizeof(*buf));
   buf->st_mtim = filetime_to_timespec(basic.LastWriteTime);
   buf->st_atim = filetime_to_timespec(basic.LastAccessTime);
@@ -168,18 +164,18 @@ inline int stat_handle(HANDLE h, StatT* buf) {
   if (basic.FileAttributes & FILE_ATTRIBUTE_REPARSE_POINT) {
     FILE_ATTRIBUTE_TAG_INFO tag;
     if (!GetFileInformationByHandleEx(h, FileAttributeTagInfo, &tag, sizeof(tag)))
-      return set_errno();
+      return -1;
     if (tag.ReparseTag == IO_REPARSE_TAG_SYMLINK)
       buf->st_mode = (buf->st_mode & ~_S_IFMT) | _S_IFLNK;
   }
   FILE_STANDARD_INFO standard;
   if (!GetFileInformationByHandleEx(h, FileStandardInfo, &standard, sizeof(standard)))
-    return set_errno();
+    return -1;
   buf->st_nlink = standard.NumberOfLinks;
   buf->st_size  = standard.EndOfFile.QuadPart;
   BY_HANDLE_FILE_INFORMATION info;
   if (!GetFileInformationByHandle(h, &info))
-    return set_errno();
+    return -1;
   buf->st_dev = info.dwVolumeSerialNumber;
   memcpy(&buf->st_ino.id[0], &info.nFileIndexHigh, 4);
   memcpy(&buf->st_ino.id[4], &info.nFileIndexLow, 4);
@@ -189,7 +185,7 @@ inline int stat_handle(HANDLE h, StatT* buf) {
 inline int stat_file(const wchar_t* path, StatT* buf, DWORD flags) {
   WinHandle h(path, FILE_READ_ATTRIBUTES, flags);
   if (!h)
-    return set_errno();
+    return -1;
   int ret = stat_handle(h, buf);
   return ret;
 }
@@ -206,7 +202,7 @@ inline int fstat(int fd, StatT* buf) {
 inline int mkdir(const wchar_t* path, int permissions) {
   (void)permissions;
   if (!CreateDirectoryW(path, nullptr))
-    return set_errno();
+    return -1;
   return 0;
 }
 
@@ -219,10 +215,10 @@ inline int symlink_file_dir(const wchar_t* oldname, const wchar_t* newname, bool
     return 0;
   int e = GetLastError();
   if (e != ERROR_INVALID_PARAMETER)
-    return set_errno(e);
+    return -1;
   if (CreateSymbolicLinkW(newname, oldname, flags))
     return 0;
-  return set_errno();
+  return -1;
 }
 
 inline int symlink_file(const wchar_t* oldname, const wchar_t* newname) {
@@ -236,17 +232,17 @@ inline int symlink_dir(const wchar_t* oldname, const wchar_t* newname) {
 inline int link(const wchar_t* oldname, const wchar_t* newname) {
   if (CreateHardLinkW(newname, oldname, nullptr))
     return 0;
-  return set_errno();
+  return -1;
 }
 
 inline int remove(const wchar_t* path) {
   detail::WinHandle h(path, DELETE, FILE_FLAG_OPEN_REPARSE_POINT);
   if (!h)
-    return set_errno();
+    return -1;
   FILE_DISPOSITION_INFO info;
   info.DeleteFile = TRUE;
   if (!SetFileInformationByHandle(h, FileDispositionInfo, &info, sizeof(info)))
-    return set_errno();
+    return -1;
   return 0;
 }
 
@@ -254,9 +250,9 @@ inline int truncate_handle(HANDLE h, off_t length) {
   LARGE_INTEGER size_param;
   size_param.QuadPart = length;
   if (!SetFilePointerEx(h, size_param, 0, FILE_BEGIN))
-    return set_errno();
+    return -1;
   if (!SetEndOfFile(h))
-    return set_errno();
+    return -1;
   return 0;
 }
 
@@ -268,19 +264,19 @@ inline int ftruncate(int fd, off_t length) {
 inline int truncate(const wchar_t* path, off_t length) {
   detail::WinHandle h(path, GENERIC_WRITE, 0);
   if (!h)
-    return set_errno();
+    return -1;
   return truncate_handle(h, length);
 }
 
 inline int rename(const wchar_t* from, const wchar_t* to) {
   if (!(MoveFileExW(from, to, MOVEFILE_COPY_ALLOWED | MOVEFILE_REPLACE_EXISTING | MOVEFILE_WRITE_THROUGH)))
-    return set_errno();
+    return -1;
   return 0;
 }
 
 inline int chdir(const wchar_t* path) {
   if (!SetCurrentDirectoryW(path))
-    return set_errno();
+    return -1;
   return 0;
 }
 
@@ -300,7 +296,7 @@ inline int statvfs(const wchar_t* p, StatVFS* buf) {
       break;
     path parent = dir.parent_path();
     if (parent == dir) {
-      errno = ENOENT;
+      SetLastError(ERROR_PATH_NOT_FOUND);
       return -1;
     }
     dir = parent;
@@ -308,7 +304,7 @@ inline int statvfs(const wchar_t* p, StatVFS* buf) {
   ULARGE_INTEGER free_bytes_available_to_caller, total_number_of_bytes, total_number_of_free_bytes;
   if (!GetDiskFreeSpaceExW(
           dir.c_str(), &free_bytes_available_to_caller, &total_number_of_bytes, &total_number_of_free_bytes))
-    return set_errno();
+    return -1;
   buf->f_frsize = 1;
   buf->f_blocks = total_number_of_bytes.QuadPart;
   buf->f_bfree  = total_number_of_free_bytes.QuadPart;
@@ -330,7 +326,6 @@ inline wchar_t* getcwd([[maybe_unused]] wchar_t* in_buf, [[maybe_unused]] size_t
     retval = GetCurrentDirectoryW(buff_size, buff.get());
   }
   if (!retval) {
-    set_errno();
     return nullptr;
   }
   return buff.release();
@@ -342,7 +337,6 @@ inline wchar_t* realpath(const wchar_t* path, [[maybe_unused]] wchar_t* resolved
 
   WinHandle h(path, FILE_READ_ATTRIBUTES, 0);
   if (!h) {
-    set_errno();
     return nullptr;
   }
   size_t buff_size = MAX_PATH + 10;
@@ -354,7 +348,6 @@ inline wchar_t* realpath(const wchar_t* path, [[maybe_unused]] wchar_t* resolved
     retval = GetFinalPathNameByHandleW(h, buff.get(), buff_size, FILE_NAME_NORMALIZED | VOLUME_NAME_DOS);
   }
   if (!retval) {
-    set_errno();
     return nullptr;
   }
   wchar_t* ptr = buff.get();
@@ -376,20 +369,20 @@ using ModeT = int;
 inline int fchmod_handle(HANDLE h, int perms) {
   FILE_BASIC_INFO basic;
   if (!GetFileInformationByHandleEx(h, FileBasicInfo, &basic, sizeof(basic)))
-    return set_errno();
+    return -1;
   DWORD orig_attributes = basic.FileAttributes;
   basic.FileAttributes &= ~FILE_ATTRIBUTE_READONLY;
   if ((perms & 0222) == 0)
     basic.FileAttributes |= FILE_ATTRIBUTE_READONLY;
   if (basic.FileAttributes != orig_attributes && !SetFileInformationByHandle(h, FileBasicInfo, &basic, sizeof(basic)))
-    return set_errno();
+    return -1;
   return 0;
 }
 
 inline int fchmodat(int /*fd*/, const wchar_t* path, int perms, int flag) {
   DWORD attributes = GetFileAttributesW(path);
   if (attributes == INVALID_FILE_ATTRIBUTES)
-    return set_errno();
+    return -1;
   if (attributes & FILE_ATTRIBUTE_REPARSE_POINT && !(flag & AT_SYMLINK_NOFOLLOW)) {
     // If the file is a symlink, and we are supposed to operate on the target
     // of the symlink, we need to open a handle to it, without the
@@ -397,7 +390,7 @@ inline int fchmodat(int /*fd*/, const wchar_t* path, int perms, int flag) {
     // symlink, and operate on it via the handle.
     detail::WinHandle h(path, FILE_READ_ATTRIBUTES | FILE_WRITE_ATTRIBUTES, 0);
     if (!h)
-      return set_errno();
+      return -1;
     return fchmod_handle(h, perms);
   } else {
     // For a non-symlink, or if operating on the symlink itself instead of
@@ -407,7 +400,7 @@ inline int fchmodat(int /*fd*/, const wchar_t* path, int perms, int flag) {
     if ((perms & 0222) == 0)
       attributes |= FILE_ATTRIBUTE_READONLY;
     if (attributes != orig_attributes && !SetFileAttributesW(path, attributes))
-      return set_errno();
+      return -1;
   }
   return 0;
 }
@@ -424,18 +417,18 @@ inline SSizeT readlink(const wchar_t* path, wchar_t* ret_buf, size_t bufsize) {
   uint8_t buf[MAXIMUM_REPARSE_DATA_BUFFER_SIZE];
   detail::WinHandle h(path, FILE_READ_ATTRIBUTES, FILE_FLAG_OPEN_REPARSE_POINT);
   if (!h)
-    return set_errno();
+    return -1;
   DWORD out;
   if (!DeviceIoControl(h, FSCTL_GET_REPARSE_POINT, nullptr, 0, buf, sizeof(buf), &out, 0))
-    return set_errno();
+    return -1;
   const auto* reparse    = reinterpret_cast<LIBCPP_REPARSE_DATA_BUFFER*>(buf);
   size_t path_buf_offset = offsetof(LIBCPP_REPARSE_DATA_BUFFER, SymbolicLinkReparseBuffer.PathBuffer[0]);
   if (out < path_buf_offset) {
-    errno = EINVAL;
+    SetLastError(ERROR_REPARSE_TAG_INVALID);
     return -1;
   }
   if (reparse->ReparseTag != IO_REPARSE_TAG_SYMLINK) {
-    errno = EINVAL;
+    SetLastError(ERROR_REPARSE_TAG_INVALID);
     return -1;
   }
   const auto& symlink = reparse->SymbolicLinkReparseBuffer;
@@ -449,11 +442,11 @@ inline SSizeT readlink(const wchar_t* path, wchar_t* ret_buf, size_t bufsize) {
   }
   // name_offset/length are expressed in bytes, not in wchar_t
   if (path_buf_offset + name_offset + name_length > out) {
-    errno = EINVAL;
+    SetLastError(ERROR_REPARSE_TAG_INVALID);
     return -1;
   }
   if (name_length / sizeof(wchar_t) > bufsize) {
-    errno = ENOMEM;
+    SetLastError(ERROR_NOT_ENOUGH_MEMORY);
     return -1;
   }
   memcpy(ret_buf, &symlink.PathBuffer[name_offset / sizeof(wchar_t)], name_length);
diff --git a/libcxx/src/include/overridable_function.h b/libcxx/src/include/overridable_function.h
index 6c70f6242ddd..7372e347831b 100644
--- a/libcxx/src/include/overridable_function.h
+++ b/libcxx/src/include/overridable_function.h
@@ -29,106 +29,81 @@
 // This is a low-level utility which does not work on all platforms, since it needs
 // to make assumptions about the object file format in use. Furthermore, it requires
 // the "base definition" of the function (the one we want to check whether it has been
-// overridden) to be annotated with the _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro.
+// overridden) to be defined using the _LIBCPP_OVERRIDABLE_FUNCTION macro.
 //
 // This currently works with Mach-O files (used on Darwin) and with ELF files (used on Linux
 // and others). On platforms where we know how to implement this detection, the macro
 // _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION is defined to 1, and it is defined to 0 on
-// other platforms. The _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro is defined to
-// nothing on unsupported platforms so that it can be used to decorate functions regardless
-// of whether detection is actually supported.
+// other platforms. The _LIBCPP_OVERRIDABLE_FUNCTION macro expands to regular function
+// definition on unsupported platforms so that it can be used to decorate functions
+// regardless of whether detection is actually supported.
 //
 // How does this work?
 // -------------------
 //
 // Let's say we want to check whether a weak function `f` has been overridden by the user.
-// The general mechanism works by placing `f`'s definition (in the libc++ built library)
-// inside a special section, which we do using the `__section__` attribute via the
-// _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro.
+// The general mechanism works by defining a symbol `f_impl__` and a weak alias `f` via the
+// _LIBCPP_OVERRIDABLE_FUNCTION macro.
 //
 // Then, when comes the time to check whether the function has been overridden, we take
-// the address of the function and we check whether it falls inside the special function
-// we created. This can be done by finding pointers to the start and the end of the section
-// (which is done differently for ELF and Mach-O), and then checking whether `f` falls
-// within those bounds. If it falls within those bounds, then `f` is still inside the
-// special section and so it is the version we defined in the libc++ built library, i.e.
-// it was not overridden. Otherwise, it was overridden by the user because it falls
-// outside of the section.
+// the address of the function `f` and we check whether it is different from `f_impl__`.
+// If so it means the function was overriden by the user.
 //
 // Important note
 // --------------
 //
-// This mechanism should never be used outside of the libc++ built library. In particular,
-// attempting to use this within the libc++ headers will not work at all because we don't
-// want to be defining special sections inside user's executables which use our headers.
+// This mechanism should never be used outside of the libc++ built library. Functions defined
+// with this macro must be defined at global scope.
 //
 
 #if defined(_LIBCPP_OBJECT_FORMAT_MACHO)
 
-#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
-#  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE                                                                 \
-    __attribute__((__section__("__TEXT,__lcxx_override,regular,pure_instructions")))
-
 _LIBCPP_BEGIN_NAMESPACE_STD
-template <class _Ret, class... _Args>
-_LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept {
-  // Declare two dummy bytes and give them these special `__asm` values. These values are
-  // defined by the linker, which means that referring to `&__lcxx_override_start` will
-  // effectively refer to the address where the section starts (and same for the end).
-  extern char __lcxx_override_start __asm("section$start$__TEXT$__lcxx_override");
-  extern char __lcxx_override_end __asm("section$end$__TEXT$__lcxx_override");
-
-  // Now get a uintptr_t out of these locations, and out of the function pointer.
-  uintptr_t __start = reinterpret_cast<uintptr_t>(&__lcxx_override_start);
-  uintptr_t __end   = reinterpret_cast<uintptr_t>(&__lcxx_override_end);
-  uintptr_t __ptr   = reinterpret_cast<uintptr_t>(__fptr);
-
-#  if __has_feature(ptrauth_calls)
-  // We must pass a void* to ptrauth_strip since it only accepts a pointer type. Also, in particular,
-  // we must NOT pass a function pointer, otherwise we will strip the function pointer, and then attempt
-  // to authenticate and re-sign it when casting it to a uintptr_t again, which will fail because we just
-  // stripped the function pointer. See rdar://122927845.
-  __ptr = reinterpret_cast<uintptr_t>(ptrauth_strip(reinterpret_cast<void*>(__ptr), ptrauth_key_function_pointer));
-#  endif
-
-  // Finally, the function was overridden if it falls outside of the section's bounds.
-  return __ptr < __start || __ptr > __end;
-}
-_LIBCPP_END_NAMESPACE_STD
 
-// The NVPTX linker cannot create '__start/__stop' sections.
-#elif defined(_LIBCPP_OBJECT_FORMAT_ELF) && !defined(__NVPTX__)
+template <auto _Func>
+_LIBCPP_HIDE_FROM_ABI constexpr bool __is_function_overridden();
 
-#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
-#  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE __attribute__((__section__("__lcxx_override")))
+_LIBCPP_END_NAMESPACE_STD
 
-// This is very similar to what we do for Mach-O above. The ELF linker will implicitly define
-// variables with those names corresponding to the start and the end of the section.
-//
-// See https://stackoverflow.com/questions/16552710/how-do-you-get-the-start-and-end-addresses-of-a-custom-elf-section
-extern char __start___lcxx_override;
-extern char __stop___lcxx_override;
+#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
+#  define _LIBCPP_OVERRIDABLE_FUNCTION(symbol, type, name, arglist)                                                    \
+    static __attribute__((used)) type symbol##_impl__ arglist __asm__("_" _LIBCPP_TOSTRING(symbol));                   \
+    __asm__(".globl _" _LIBCPP_TOSTRING(symbol));                                                                      \
+    __asm__(".weak_definition _" _LIBCPP_TOSTRING(symbol));                                                            \
+    extern __typeof(symbol##_impl__) name __attribute__((weak_import));                                                \
+    _LIBCPP_BEGIN_NAMESPACE_STD                                                                                        \
+    template <>                                                                                                        \
+    inline bool __is_function_overridden<static_cast<type(*) arglist>(name)>() {                                       \
+      return static_cast<type(*) arglist>(name) != symbol##_impl__;                                                    \
+    }                                                                                                                  \
+    _LIBCPP_END_NAMESPACE_STD                                                                                          \
+    static type symbol##_impl__ arglist
+
+#elif defined(_LIBCPP_OBJECT_FORMAT_ELF)
 
 _LIBCPP_BEGIN_NAMESPACE_STD
-template <class _Ret, class... _Args>
-_LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept {
-  uintptr_t __start = reinterpret_cast<uintptr_t>(&__start___lcxx_override);
-  uintptr_t __end   = reinterpret_cast<uintptr_t>(&__stop___lcxx_override);
-  uintptr_t __ptr   = reinterpret_cast<uintptr_t>(__fptr);
-
-#  if __has_feature(ptrauth_calls)
-  // We must pass a void* to ptrauth_strip since it only accepts a pointer type. See full explanation above.
-  __ptr = reinterpret_cast<uintptr_t>(ptrauth_strip(reinterpret_cast<void*>(__ptr), ptrauth_key_function_pointer));
-#  endif
-
-  return __ptr < __start || __ptr > __end;
-}
+
+template <auto _Func>
+_LIBCPP_HIDE_FROM_ABI constexpr bool __is_function_overridden();
+
 _LIBCPP_END_NAMESPACE_STD
 
+#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
+#  define _LIBCPP_OVERRIDABLE_FUNCTION(symbol, type, name, arglist)                                                    \
+    static type symbol##_impl__ arglist __asm__(_LIBCPP_TOSTRING(symbol##_impl__));                                    \
+    [[gnu::weak, gnu::alias(_LIBCPP_TOSTRING(symbol##_impl__))]] type name arglist;                                    \
+    _LIBCPP_BEGIN_NAMESPACE_STD                                                                                        \
+    template <>                                                                                                        \
+    inline bool __is_function_overridden<static_cast<type(*) arglist>(name)>() {                                       \
+      return static_cast<type(*) arglist>(name) != symbol##_impl__;                                                    \
+    }                                                                                                                  \
+    _LIBCPP_END_NAMESPACE_STD                                                                                          \
+    static type symbol##_impl__ arglist
+
 #else
 
 #  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 0
-#  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE /* nothing */
+#  define _LIBCPP_OVERRIDABLE_FUNCTION(symbol, type, name, arglist) _LIBCPP_WEAK type name arglist
 
 #endif
 
diff --git a/libcxx/src/new.cpp b/libcxx/src/new.cpp
index e010fe4c4f19..b14b52248df3 100644
--- a/libcxx/src/new.cpp
+++ b/libcxx/src/new.cpp
@@ -43,7 +43,7 @@ static void* operator_new_impl(std::size_t size) {
   return p;
 }
 
-_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void* operator new(std::size_t size) _THROW_BAD_ALLOC {
+_LIBCPP_OVERRIDABLE_FUNCTION(_Znwm, void*, operator new, (std::size_t size)) _THROW_BAD_ALLOC {
   void* p = operator_new_impl(size);
   if (p == nullptr)
     __throw_bad_alloc_shim();
@@ -54,7 +54,7 @@ _LIBCPP_WEAK void* operator new(size_t size, const std::nothrow_t&) noexcept {
 #  if !_LIBCPP_HAS_EXCEPTIONS
 #    if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
   _LIBCPP_ASSERT_SHIM(
-      !std::__is_function_overridden(static_cast<void* (*)(std::size_t)>(&operator new)),
+      !std::__is_function_overridden<static_cast<void* (*)(std::size_t)>(&operator new)>(),
       "libc++ was configured with exceptions disabled and `operator new(size_t)` has been overridden, "
       "but `operator new(size_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new(size_t, nothrow_t)` must call `operator new(size_t)`, which will terminate in case "
@@ -74,7 +74,7 @@ _LIBCPP_WEAK void* operator new(size_t size, const std::nothrow_t&) noexcept {
 #  endif
 }
 
-_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void* operator new[](size_t size) _THROW_BAD_ALLOC {
+_LIBCPP_OVERRIDABLE_FUNCTION(_Znam, void*, operator new[], (size_t size)) _THROW_BAD_ALLOC {
   return ::operator new(size);
 }
 
@@ -82,7 +82,7 @@ _LIBCPP_WEAK void* operator new[](size_t size, const std::nothrow_t&) noexcept {
 #  if !_LIBCPP_HAS_EXCEPTIONS
 #    if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
   _LIBCPP_ASSERT_SHIM(
-      !std::__is_function_overridden(static_cast<void* (*)(std::size_t)>(&operator new[])),
+      !std::__is_function_overridden<static_cast<void* (*)(std::size_t)>(&operator new[])>(),
       "libc++ was configured with exceptions disabled and `operator new[](size_t)` has been overridden, "
       "but `operator new[](size_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new[](size_t, nothrow_t)` must call `operator new[](size_t)`, which will terminate in case "
@@ -136,8 +136,8 @@ static void* operator_new_aligned_impl(std::size_t size, std::align_val_t alignm
   return p;
 }
 
-_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void*
-operator new(std::size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
+_LIBCPP_OVERRIDABLE_FUNCTION(_ZnwmSt11align_val_t, void*, operator new, (std::size_t size, std::align_val_t alignment))
+_THROW_BAD_ALLOC {
   void* p = operator_new_aligned_impl(size, alignment);
   if (p == nullptr)
     __throw_bad_alloc_shim();
@@ -148,7 +148,7 @@ _LIBCPP_WEAK void* operator new(size_t size, std::align_val_t alignment, const s
 #    if !_LIBCPP_HAS_EXCEPTIONS
 #      if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
   _LIBCPP_ASSERT_SHIM(
-      !std::__is_function_overridden(static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new)),
+      !std::__is_function_overridden<static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new)>(),
       "libc++ was configured with exceptions disabled and `operator new(size_t, align_val_t)` has been overridden, "
       "but `operator new(size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new(size_t, align_val_t, nothrow_t)` must call `operator new(size_t, align_val_t)`, which will "
@@ -168,16 +168,14 @@ _LIBCPP_WEAK void* operator new(size_t size, std::align_val_t alignment, const s
 #    endif
 }
 
-_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void*
-operator new[](size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
-  return ::operator new(size, alignment);
-}
+_LIBCPP_OVERRIDABLE_FUNCTION(_ZnamSt11align_val_t, void*, operator new[], (size_t size, std::align_val_t alignment))
+_THROW_BAD_ALLOC { return ::operator new(size, alignment); }
 
 _LIBCPP_WEAK void* operator new[](size_t size, std::align_val_t alignment, const std::nothrow_t&) noexcept {
 #    if !_LIBCPP_HAS_EXCEPTIONS
 #      if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
   _LIBCPP_ASSERT_SHIM(
-      !std::__is_function_overridden(static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new[])),
+      !std::__is_function_overridden<static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new[])>(),
       "libc++ was configured with exceptions disabled and `operator new[](size_t, align_val_t)` has been overridden, "
       "but `operator new[](size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new[](size_t, align_val_t, nothrow_t)` must call `operator new[](size_t, align_val_t)`, which will "
diff --git a/libcxx/src/print.cpp b/libcxx/src/print.cpp
index 37b1fc00cd7c..4937aafe8417 100644
--- a/libcxx/src/print.cpp
+++ b/libcxx/src/print.cpp
@@ -51,7 +51,7 @@ __write_to_windows_console([[maybe_unused]] FILE* __stream, [[maybe_unused]] wst
                     __view.size(),
                     nullptr,
                     nullptr) == 0) {
-    __throw_system_error(filesystem::detail::make_windows_error(GetLastError()), "failed to write formatted output");
+    __throw_system_error(filesystem::detail::get_last_error(), "failed to write formatted output");
   }
 }
 #  endif // _LIBCPP_HAS_WIDE_CHARACTERS
diff --git a/libcxx/src/system_error.cpp b/libcxx/src/system_error.cpp
index d555bca995c4..d5ec73084f63 100644
--- a/libcxx/src/system_error.cpp
+++ b/libcxx/src/system_error.cpp
@@ -14,6 +14,7 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <optional>
 #include <string.h>
 #include <string>
 #include <system_error>
@@ -24,8 +25,123 @@
 #  include <android/api-level.h>
 #endif
 
+#if defined(_LIBCPP_WIN32API)
+#  include <windows.h>
+#  include <winerror.h>
+#endif
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#if defined(_LIBCPP_WIN32API)
+
+namespace {
+std::optional<errc> __win_err_to_errc(int err) {
+  switch (err) {
+  case ERROR_ACCESS_DENIED:
+    return errc::permission_denied;
+  case ERROR_ALREADY_EXISTS:
+    return errc::file_exists;
+  case ERROR_BAD_NETPATH:
+    return errc::no_such_file_or_directory;
+  case ERROR_BAD_PATHNAME:
+    return errc::no_such_file_or_directory;
+  case ERROR_BAD_UNIT:
+    return errc::no_such_device;
+  case ERROR_BROKEN_PIPE:
+    return errc::broken_pipe;
+  case ERROR_BUFFER_OVERFLOW:
+    return errc::filename_too_long;
+  case ERROR_BUSY:
+    return errc::device_or_resource_busy;
+  case ERROR_BUSY_DRIVE:
+    return errc::device_or_resource_busy;
+  case ERROR_CANNOT_MAKE:
+    return errc::permission_denied;
+  case ERROR_CANTOPEN:
+    return errc::io_error;
+  case ERROR_CANTREAD:
+    return errc::io_error;
+  case ERROR_CANTWRITE:
+    return errc::io_error;
+  case ERROR_CURRENT_DIRECTORY:
+    return errc::permission_denied;
+  case ERROR_DEV_NOT_EXIST:
+    return errc::no_such_device;
+  case ERROR_DEVICE_IN_USE:
+    return errc::device_or_resource_busy;
+  case ERROR_DIR_NOT_EMPTY:
+    return errc::directory_not_empty;
+  case ERROR_DIRECTORY:
+    return errc::invalid_argument;
+  case ERROR_DISK_FULL:
+    return errc::no_space_on_device;
+  case ERROR_FILE_EXISTS:
+    return errc::file_exists;
+  case ERROR_FILE_NOT_FOUND:
+    return errc::no_such_file_or_directory;
+  case ERROR_HANDLE_DISK_FULL:
+    return errc::no_space_on_device;
+  case ERROR_INVALID_ACCESS:
+    return errc::permission_denied;
+  case ERROR_INVALID_DRIVE:
+    return errc::no_such_device;
+  case ERROR_INVALID_FUNCTION:
+    return errc::function_not_supported;
+  case ERROR_INVALID_HANDLE:
+    return errc::invalid_argument;
+  case ERROR_INVALID_NAME:
+    return errc::no_such_file_or_directory;
+  case ERROR_INVALID_PARAMETER:
+    return errc::invalid_argument;
+  case ERROR_LOCK_VIOLATION:
+    return errc::no_lock_available;
+  case ERROR_LOCKED:
+    return errc::no_lock_available;
+  case ERROR_NEGATIVE_SEEK:
+    return errc::invalid_argument;
+  case ERROR_NOACCESS:
+    return errc::permission_denied;
+  case ERROR_NOT_ENOUGH_MEMORY:
+    return errc::not_enough_memory;
+  case ERROR_NOT_READY:
+    return errc::resource_unavailable_try_again;
+  case ERROR_NOT_SAME_DEVICE:
+    return errc::cross_device_link;
+  case ERROR_NOT_SUPPORTED:
+    return errc::not_supported;
+  case ERROR_OPEN_FAILED:
+    return errc::io_error;
+  case ERROR_OPEN_FILES:
+    return errc::device_or_resource_busy;
+  case ERROR_OPERATION_ABORTED:
+    return errc::operation_canceled;
+  case ERROR_OUTOFMEMORY:
+    return errc::not_enough_memory;
+  case ERROR_PATH_NOT_FOUND:
+    return errc::no_such_file_or_directory;
+  case ERROR_READ_FAULT:
+    return errc::io_error;
+  case ERROR_REPARSE_TAG_INVALID:
+    return errc::invalid_argument;
+  case ERROR_RETRY:
+    return errc::resource_unavailable_try_again;
+  case ERROR_SEEK:
+    return errc::io_error;
+  case ERROR_SHARING_VIOLATION:
+    return errc::permission_denied;
+  case ERROR_TOO_MANY_OPEN_FILES:
+    return errc::too_many_files_open;
+  case ERROR_WRITE_FAULT:
+    return errc::io_error;
+  case ERROR_WRITE_PROTECT:
+    return errc::permission_denied;
+  default:
+    return {};
+  }
+}
+} // namespace
+#endif
+
 namespace {
 #if _LIBCPP_HAS_THREADS
 
@@ -157,19 +273,52 @@ public:
 const char* __system_error_category::name() const noexcept { return "system"; }
 
 string __system_error_category::message(int ev) const {
-#ifdef _LIBCPP_ELAST
+#ifdef _LIBCPP_WIN32API
+  std::string result;
+  char* str               = nullptr;
+  unsigned long num_chars = ::FormatMessageA(
+      FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
+      nullptr,
+      ev,
+      0,
+      reinterpret_cast<char*>(&str),
+      0,
+      nullptr);
+  auto is_whitespace = [](char ch) { return ch == '\n' || ch == '\r' || ch == ' '; };
+  while (num_chars > 0 && is_whitespace(str[num_chars - 1]))
+    --num_chars;
+
+  if (num_chars)
+    result = std::string(str, num_chars);
+  else
+    result = "Unknown error";
+
+  LocalFree(str);
+  return result;
+#else
+#  ifdef _LIBCPP_ELAST
   if (ev > _LIBCPP_ELAST)
     return string("unspecified system_category error");
-#endif // _LIBCPP_ELAST
+#  endif // _LIBCPP_ELAST
   return __do_message::message(ev);
+#endif
 }
 
 error_condition __system_error_category::default_error_condition(int ev) const noexcept {
-#ifdef _LIBCPP_ELAST
+#ifdef _LIBCPP_WIN32API
+  // Remap windows error codes to generic error codes if possible.
+  if (ev == 0)
+    return error_condition(0, generic_category());
+  if (auto maybe_errc = __win_err_to_errc(ev))
+    return error_condition(static_cast<int>(*maybe_errc), generic_category());
+  return error_condition(ev, system_category());
+#else
+#  ifdef _LIBCPP_ELAST
   if (ev > _LIBCPP_ELAST)
     return error_condition(ev, system_category());
-#endif // _LIBCPP_ELAST
+#  endif // _LIBCPP_ELAST
   return error_condition(ev, generic_category());
+#endif
 }
 
 const error_category& system_category() noexcept {
@@ -213,7 +362,7 @@ system_error::~system_error() noexcept {}
 
 void __throw_system_error(int ev, const char* what_arg) {
 #if _LIBCPP_HAS_EXCEPTIONS
-  std::__throw_system_error(error_code(ev, system_category()), what_arg);
+  std::__throw_system_error(error_code(ev, generic_category()), what_arg);
 #else
   // The above could also handle the no-exception case, but for size, avoid referencing system_category() unnecessarily.
   _LIBCPP_VERBOSE_ABORT(
diff --git a/libcxx/test/benchmarks/numeric/gcd.bench.cpp b/libcxx/test/benchmarks/numeric/gcd.bench.cpp
index abbc7e9dd04f..ca5fed59463a 100644
--- a/libcxx/test/benchmarks/numeric/gcd.bench.cpp
+++ b/libcxx/test/benchmarks/numeric/gcd.bench.cpp
@@ -25,7 +25,7 @@ static std::array<T, 1000> generate(std::uniform_int_distribution<T> distributio
 
 static void bm_gcd_random(benchmark::State& state) {
   std::array data = generate<int>();
-  while (state.KeepRunningBatch(data.size()))
+  while (state.KeepRunningBatch(data.size() * data.size()))
     for (auto v0 : data)
       for (auto v1 : data)
         benchmark::DoNotOptimize(std::gcd(v0, v1));
diff --git a/libcxx/test/libcxx/clang_tidy.gen.py b/libcxx/test/libcxx/clang_tidy.gen.py
index 0db9c0d14b19..06f277e901d3 100644
--- a/libcxx/test/libcxx/clang_tidy.gen.py
+++ b/libcxx/test/libcxx/clang_tidy.gen.py
@@ -33,8 +33,7 @@ for header in public_headers:
 {lit_header_undeprecations.get(header, '')}
 
 // TODO: run clang-tidy with modules enabled once they are supported
-// RUN: %{{clang-tidy}} %s --warnings-as-errors=* -header-filter=.* --checks='-*,libcpp-*' --load=%{{test-tools-dir}}/clang_tidy_checks/libcxx-tidy.plugin -- %{{compile_flags}} -fno-modules
-// RUN: %{{clang-tidy}} %s --warnings-as-errors=* -header-filter=.* --config-file=%{{libcxx-dir}}/.clang-tidy -- -Wweak-vtables %{{compile_flags}} -fno-modules
+// RUN: %{{clang-tidy}} %s --warnings-as-errors=* -header-filter=.* --config-file=%{{libcxx-dir}}/.clang-tidy --load=%{{test-tools-dir}}/clang_tidy_checks/libcxx-tidy.plugin -- -Wweak-vtables %{{compile_flags}} -fno-modules
 
 #include <{header}>
 """)
diff --git a/libcxx/test/libcxx/containers/sequences/forwardlist/assert.pass.cpp b/libcxx/test/libcxx/containers/sequences/forwardlist/assert.pass.cpp
new file mode 100644
index 000000000000..6d1748e64502
--- /dev/null
+++ b/libcxx/test/libcxx/containers/sequences/forwardlist/assert.pass.cpp
@@ -0,0 +1,47 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <forward_list>
+
+// Test hardening assertions for std::forward_list.
+
+// REQUIRES: has-unix-headers
+// REQUIRES: libcpp-hardening-mode={{extensive|debug}}
+// UNSUPPORTED: c++03
+// XFAIL: libcpp-hardening-mode=debug && availability-verbose_abort-missing
+
+#include <forward_list>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  { // Default-constructed list.
+    std::forward_list<int> c;
+    const auto& const_c = c;
+    TEST_LIBCPP_ASSERT_FAILURE(c.front(), "forward_list::front called on an empty list");
+    TEST_LIBCPP_ASSERT_FAILURE(const_c.front(), "forward_list::front called on an empty list");
+    TEST_LIBCPP_ASSERT_FAILURE(c.pop_front(), "forward_list::pop_front called on an empty list");
+  }
+
+  { // Non-empty list becomes empty.
+    std::forward_list<int> c;
+    const auto& const_c = c;
+    c.push_front(1);
+
+    // Check that there's no assertion on valid access.
+    (void)c.front();
+    (void)const_c.front();
+
+    c.pop_front();
+    TEST_LIBCPP_ASSERT_FAILURE(c.pop_front(), "forward_list::pop_front called on an empty list");
+    TEST_LIBCPP_ASSERT_FAILURE(c.front(), "forward_list::front called on an empty list");
+    TEST_LIBCPP_ASSERT_FAILURE(const_c.front(), "forward_list::front called on an empty list");
+  }
+
+  return 0;
+}
diff --git a/libcxx/test/libcxx/containers/sequences/vector.bool/assert.pass.cpp b/libcxx/test/libcxx/containers/sequences/vector.bool/assert.pass.cpp
new file mode 100644
index 000000000000..41badad8f569
--- /dev/null
+++ b/libcxx/test/libcxx/containers/sequences/vector.bool/assert.pass.cpp
@@ -0,0 +1,63 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <vector>
+
+// Test hardening assertions for std::vector<bool>.
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: libcpp-hardening-mode=none
+// UNSUPPORTED: c++03
+// XFAIL: libcpp-hardening-mode=debug && availability-verbose_abort-missing
+
+#include <vector>
+
+#include "check_assertion.h"
+#include "min_allocator.h"
+
+template <class Allocator>
+void test() {
+  std::vector<bool, Allocator> c;
+  TEST_LIBCPP_ASSERT_FAILURE(c.front(), "vector<bool>::front() called on an empty vector");
+  TEST_LIBCPP_ASSERT_FAILURE(c.back(), "vector<bool>::back() called on an empty vector");
+  TEST_LIBCPP_ASSERT_FAILURE(c[0], "vector<bool>::operator[] index out of bounds");
+  TEST_LIBCPP_ASSERT_FAILURE(c.pop_back(), "vector<bool>::pop_back called on an empty vector");
+
+  // Repeat the test with a const reference to test the const overloads.
+  {
+    const std::vector<bool, Allocator>& cc = c;
+    TEST_LIBCPP_ASSERT_FAILURE(cc.front(), "vector<bool>::front() called on an empty vector");
+    TEST_LIBCPP_ASSERT_FAILURE(cc.back(), "vector<bool>::back() called on an empty vector");
+    TEST_LIBCPP_ASSERT_FAILURE(cc[0], "vector<bool>::operator[] index out of bounds");
+  }
+
+  c.push_back(true);
+  c.push_back(false);
+  c.push_back(true);
+  TEST_LIBCPP_ASSERT_FAILURE(c[3], "vector<bool>::operator[] index out of bounds");
+  TEST_LIBCPP_ASSERT_FAILURE(c[100], "vector<bool>::operator[] index out of bounds");
+
+  // Repeat the test with a const reference to test the const overloads.
+  {
+    const std::vector<bool, Allocator>& cc = c;
+    TEST_LIBCPP_ASSERT_FAILURE(cc[3], "vector<bool>::operator[] index out of bounds");
+    TEST_LIBCPP_ASSERT_FAILURE(cc[100], "vector<bool>::operator[] index out of bounds");
+  }
+
+  TEST_LIBCPP_ASSERT_FAILURE(
+      c.erase(c.end()), "vector<bool>::erase(iterator) called with a non-dereferenceable iterator");
+  TEST_LIBCPP_ASSERT_FAILURE(
+      c.erase(c.begin() + 1, c.begin()), "vector<bool>::erase(iterator, iterator) called with an invalid range");
+}
+
+int main(int, char**) {
+  test<std::allocator<bool>>();
+  test<min_allocator<bool>>();
+
+  return 0;
+}
diff --git a/libcxx/test/libcxx/diagnostics/system_error_win_codes.pass.cpp b/libcxx/test/libcxx/diagnostics/system_error_win_codes.pass.cpp
new file mode 100644
index 000000000000..799a5b5c0b08
--- /dev/null
+++ b/libcxx/test/libcxx/diagnostics/system_error_win_codes.pass.cpp
@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: windows
+
+// Validate that system_error on windows accepts Windows' System Error Codes (as
+// used by win32 APIs and reported by GetLastError), and that they are properly
+// translated to generic conditions.
+
+#include <windows.h>
+#include <system_error>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main(int, char**) {
+  LIBCPP_ASSERT(std::error_code(ERROR_ACCESS_DENIED, std::system_category()) == std::errc::permission_denied);
+  LIBCPP_ASSERT(std::error_code(ERROR_PATH_NOT_FOUND, std::system_category()) == std::errc::no_such_file_or_directory);
+  return 0;
+}
diff --git a/libcxx/test/std/atomics/atomics.ref/exchange.pass.cpp b/libcxx/test/std/atomics/atomics.ref/exchange.pass.cpp
index cd998d46b7e8..c2afa6b8dfd0 100644
--- a/libcxx/test/std/atomics/atomics.ref/exchange.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.ref/exchange.pass.cpp
@@ -17,24 +17,47 @@
 #include <type_traits>
 
 #include "atomic_helpers.h"
+#include "test_helper.h"
 #include "test_macros.h"
 
 template <typename T>
 struct TestExchange {
   void operator()() const {
-    T x(T(1));
-    std::atomic_ref<T> const a(x);
+    {
+      T x(T(1));
+      std::atomic_ref<T> const a(x);
+
+      {
+        std::same_as<T> decltype(auto) y = a.exchange(T(2));
+        assert(y == T(1));
+        ASSERT_NOEXCEPT(a.exchange(T(2)));
+      }
+
+      {
+        std::same_as<T> decltype(auto) y = a.exchange(T(3), std::memory_order_seq_cst);
+        assert(y == T(2));
+        ASSERT_NOEXCEPT(a.exchange(T(3), std::memory_order_seq_cst));
+      }
+    }
 
+    // memory_order::release
     {
-      std::same_as<T> decltype(auto) y = a.exchange(T(2));
-      assert(y == T(1));
-      ASSERT_NOEXCEPT(a.exchange(T(2)));
+      auto exchange = [](std::atomic_ref<T> const& x, T, T new_val) {
+        x.exchange(new_val, std::memory_order::release);
+      };
+      auto load = [](std::atomic_ref<T> const& x) { return x.load(std::memory_order::acquire); };
+      test_acquire_release<T>(exchange, load);
     }
 
+    // memory_order::seq_cst
     {
-      std::same_as<T> decltype(auto) y = a.exchange(T(3), std::memory_order_seq_cst);
-      assert(y == T(2));
-      ASSERT_NOEXCEPT(a.exchange(T(3), std::memory_order_seq_cst));
+      auto exchange_no_arg     = [](std::atomic_ref<T> const& x, T, T new_val) { x.exchange(new_val); };
+      auto exchange_with_order = [](std::atomic_ref<T> const& x, T, T new_val) {
+        x.exchange(new_val, std::memory_order::seq_cst);
+      };
+      auto load = [](std::atomic_ref<T> const& x) { return x.load(); };
+      test_seq_cst<T>(exchange_no_arg, load);
+      test_seq_cst<T>(exchange_with_order, load);
     }
   }
 };
diff --git a/libcxx/test/std/containers/sequences/vector.bool/at.pass.cpp b/libcxx/test/std/containers/sequences/vector.bool/at.pass.cpp
new file mode 100644
index 000000000000..16832dd831e6
--- /dev/null
+++ b/libcxx/test/std/containers/sequences/vector.bool/at.pass.cpp
@@ -0,0 +1,125 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <vector>
+
+// reference at(size_type n); // constexpr since C++20
+
+#include <cassert>
+#include <memory>
+#include <vector>
+
+#include "min_allocator.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+#  include <stdexcept>
+#endif
+
+template <typename Allocator>
+TEST_CONSTEXPR_CXX20 void test() {
+  using C         = std::vector<bool, Allocator>;
+  using reference = typename C::reference;
+  bool a[]        = {1, 0, 1, 0, 1};
+  C v(a, a + sizeof(a) / sizeof(a[0]));
+  ASSERT_SAME_TYPE(reference, decltype(v.at(0)));
+  assert(v.at(0) == true);
+  assert(v.at(1) == false);
+  assert(v.at(2) == true);
+  assert(v.at(3) == false);
+  assert(v.at(4) == true);
+  v.at(1) = 1;
+  assert(v.at(1) == true);
+  v.at(3) = 1;
+  assert(v.at(3) == true);
+}
+
+template <typename Allocator>
+void test_exception() {
+#ifndef TEST_HAS_NO_EXCEPTIONS
+  {
+    bool a[] = {1, 0, 1, 1};
+    using C  = std::vector<bool, Allocator>;
+    C v(a, a + sizeof(a) / sizeof(a[0]));
+
+    try {
+      TEST_IGNORE_NODISCARD v.at(4);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+
+    try {
+      TEST_IGNORE_NODISCARD v.at(5);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+
+    try {
+      TEST_IGNORE_NODISCARD v.at(6);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+
+    try {
+      using size_type = typename C::size_type;
+      TEST_IGNORE_NODISCARD v.at(static_cast<size_type>(-1));
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+  }
+
+  {
+    std::vector<bool, Allocator> v;
+    try {
+      TEST_IGNORE_NODISCARD v.at(0);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+  }
+#endif
+}
+
+TEST_CONSTEXPR_CXX20 bool tests() {
+  test<std::allocator<bool> >();
+  test<min_allocator<bool> >();
+  test<test_allocator<bool> >();
+  return true;
+}
+
+void test_exceptions() {
+  test_exception<std::allocator<bool> >();
+  test_exception<min_allocator<bool> >();
+  test_exception<test_allocator<bool> >();
+}
+
+int main(int, char**) {
+  tests();
+  test_exceptions();
+
+#if TEST_STD_VER >= 20
+  static_assert(tests());
+#endif
+
+  return 0;
+}
diff --git a/libcxx/test/std/containers/sequences/vector.bool/at_const.pass.cpp b/libcxx/test/std/containers/sequences/vector.bool/at_const.pass.cpp
new file mode 100644
index 000000000000..5ed794d13f19
--- /dev/null
+++ b/libcxx/test/std/containers/sequences/vector.bool/at_const.pass.cpp
@@ -0,0 +1,121 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <vector>
+
+// const_reference at(size_type n) const; // constexpr since C++20
+
+#include <cassert>
+#include <memory>
+#include <vector>
+
+#include "min_allocator.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+#  include <stdexcept>
+#endif
+
+template <typename Allocator>
+TEST_CONSTEXPR_CXX20 void test() {
+  using C               = const std::vector<bool, Allocator>;
+  using const_reference = typename C::const_reference;
+  bool a[]              = {1, 0, 1, 0, 1};
+  C v(a, a + sizeof(a) / sizeof(a[0]));
+  ASSERT_SAME_TYPE(const_reference, decltype(v.at(0)));
+  assert(v.at(0) == true);
+  assert(v.at(1) == false);
+  assert(v.at(2) == true);
+  assert(v.at(3) == false);
+  assert(v.at(4) == true);
+}
+
+template <typename Allocator>
+void test_exception() {
+#ifndef TEST_HAS_NO_EXCEPTIONS
+  {
+    bool a[] = {1, 0, 1, 1};
+    using C  = const std::vector<bool, Allocator>;
+    C v(a, a + sizeof(a) / sizeof(a[0]));
+
+    try {
+      TEST_IGNORE_NODISCARD v.at(4);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+
+    try {
+      TEST_IGNORE_NODISCARD v.at(5);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+
+    try {
+      TEST_IGNORE_NODISCARD v.at(6);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+
+    try {
+      using size_type = typename C::size_type;
+      TEST_IGNORE_NODISCARD v.at(static_cast<size_type>(-1));
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+  }
+
+  {
+    std::vector<bool, Allocator> v;
+    try {
+      TEST_IGNORE_NODISCARD v.at(0);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+  }
+#endif
+}
+
+TEST_CONSTEXPR_CXX20 bool tests() {
+  test<std::allocator<bool> >();
+  test<min_allocator<bool> >();
+  test<test_allocator<bool> >();
+  return true;
+}
+
+void test_exceptions() {
+  test_exception<std::allocator<bool> >();
+  test_exception<min_allocator<bool> >();
+  test_exception<test_allocator<bool> >();
+}
+
+int main(int, char**) {
+  tests();
+  test_exceptions();
+
+#if TEST_STD_VER >= 20
+  static_assert(tests());
+#endif
+
+  return 0;
+}
diff --git a/libcxx/test/std/diagnostics/syserr/syserr.compare/eq_error_code_error_code.pass.cpp b/libcxx/test/std/diagnostics/syserr/syserr.compare/eq_error_code_error_code.pass.cpp
index f1f49733280b..a8b565bb0ab9 100644
--- a/libcxx/test/std/diagnostics/syserr/syserr.compare/eq_error_code_error_code.pass.cpp
+++ b/libcxx/test/std/diagnostics/syserr/syserr.compare/eq_error_code_error_code.pass.cpp
@@ -22,6 +22,10 @@
 
 #include "test_macros.h"
 
+#ifndef _WIN32
+#  define TEST_SYSTEM_CATEGORY_IS_GENERIC_CATEGORY
+#endif
+
 int main(int, char**) {
   std::error_code e_code1(5, std::generic_category());
   std::error_code e_code2(5, std::system_category());
@@ -45,7 +49,9 @@ int main(int, char**) {
   assert(e_code2 == e_code2);
   assert(e_code2 != e_code3);
   assert(e_code2 != e_code4);
+#ifdef TEST_SYSTEM_CATEGORY_IS_GENERIC_CATEGORY
   LIBCPP_ASSERT(e_code2 == e_condition1);
+#endif
   assert(e_code2 == e_condition2);
   LIBCPP_ASSERT(e_code2 != e_condition3);
   assert(e_code2 != e_condition4);
@@ -65,11 +71,15 @@ int main(int, char**) {
   assert(e_code4 == e_code4);
   LIBCPP_ASSERT(e_code4 != e_condition1);
   assert(e_code4 != e_condition2);
+#ifdef TEST_SYSTEM_CATEGORY_IS_GENERIC_CATEGORY
   LIBCPP_ASSERT(e_code4 == e_condition3);
+#endif
   assert(e_code4 == e_condition4);
 
   assert(e_condition1 == e_code1);
+#ifdef TEST_SYSTEM_CATEGORY_IS_GENERIC_CATEGORY
   LIBCPP_ASSERT(e_condition1 == e_code2);
+#endif
   assert(e_condition1 != e_code3);
   LIBCPP_ASSERT(e_condition1 != e_code4);
   assert(e_condition1 == e_condition1);
@@ -89,7 +99,9 @@ int main(int, char**) {
   assert(e_condition3 != e_code1);
   LIBCPP_ASSERT(e_condition3 != e_code2);
   assert(e_condition3 == e_code3);
+#ifdef TEST_SYSTEM_CATEGORY_IS_GENERIC_CATEGORY
   LIBCPP_ASSERT(e_condition3 == e_code4);
+#endif
   assert(e_condition3 != e_condition1);
   assert(e_condition3 != e_condition2);
   assert(e_condition3 == e_condition3);
diff --git a/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.derived/message.pass.cpp b/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.derived/message.pass.cpp
index 9f7eb42bc78d..f7f43132902f 100644
--- a/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.derived/message.pass.cpp
+++ b/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.derived/message.pass.cpp
@@ -29,8 +29,11 @@ int main(int, char**) {
   assert(!m1.empty());
   assert(!m2.empty());
   assert(!m3.empty());
+#ifndef _WIN32
+  // On windows, system_category is distinct.
   LIBCPP_ASSERT(m1 == m2);
-  assert(m1 != m3);
+#endif
+  assert(m2 != m3);
 
   return 0;
 }
diff --git a/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/system_category.pass.cpp b/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/system_category.pass.cpp
index 6ba33ba44ca4..255cbe75e2fa 100644
--- a/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/system_category.pass.cpp
+++ b/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/system_category.pass.cpp
@@ -33,7 +33,12 @@ int main(int, char**) {
   {
     const std::error_category& e_cat1 = std::system_category();
     std::error_condition e_cond       = e_cat1.default_error_condition(5);
+#ifdef _WIN32
+    // Windows' system error 5 is ERROR_ACCESS_DENIED, which maps to generic code permission_denied.
+    LIBCPP_ASSERT(e_cond.value() == static_cast<int>(std::errc::permission_denied));
+#else
     LIBCPP_ASSERT(e_cond.value() == 5);
+#endif
     LIBCPP_ASSERT(e_cond.category() == std::generic_category());
     assert(e_cat1.equivalent(5, e_cond));
 
diff --git a/libcxx/test/std/input.output/filesystems/class.directory_entry/directory_entry.obs/file_type_obs.pass.cpp b/libcxx/test/std/input.output/filesystems/class.directory_entry/directory_entry.obs/file_type_obs.pass.cpp
index 303a95a0128b..071ee7f6c891 100644
--- a/libcxx/test/std/input.output/filesystems/class.directory_entry/directory_entry.obs/file_type_obs.pass.cpp
+++ b/libcxx/test/std/input.output/filesystems/class.directory_entry/directory_entry.obs/file_type_obs.pass.cpp
@@ -172,8 +172,13 @@ static void test_with_ec_dne() {
     file_status st = status(p, status_ec);
     file_status sym_st = symlink_status(p, sym_status_ec);
     std::error_code ec = GetTestEC(2);
-    auto CheckEC = [&](std::error_code const& other_ec) {
-      bool res = ec == other_ec;
+    auto CheckEC                  = [&](std::error_code const& other_ec) {
+      // Note: we're comparing equality of the _canonicalized_ error_condition
+      // here (unlike in other tests where we expect exactly the same
+      // error_code). This is because directory_entry can construct its own
+      // generic_category error when a file doesn't exist, instead of passing
+      // through an underlying system_category error.
+      bool res = ec.default_error_condition() == other_ec.default_error_condition();
       ec = GetTestEC(2);
       return res;
     };
diff --git a/libcxx/test/std/input.output/filesystems/class.directory_entry/directory_entry.obs/status.pass.cpp b/libcxx/test/std/input.output/filesystems/class.directory_entry/directory_entry.obs/status.pass.cpp
index dd72232ee530..dec04df7ca01 100644
--- a/libcxx/test/std/input.output/filesystems/class.directory_entry/directory_entry.obs/status.pass.cpp
+++ b/libcxx/test/std/input.output/filesystems/class.directory_entry/directory_entry.obs/status.pass.cpp
@@ -44,7 +44,7 @@ static void test_basic() {
     file_status es = e.status(eec);
     assert(ps.type() == es.type());
     assert(ps.permissions() == es.permissions());
-    assert(pec == eec);
+    assert(pec.default_error_condition() == eec.default_error_condition());
   }
   for (const auto& p : TestCases) {
     const directory_entry e(p);
diff --git a/libcxx/test/std/input.output/filesystems/class.directory_entry/directory_entry.obs/symlink_status.pass.cpp b/libcxx/test/std/input.output/filesystems/class.directory_entry/directory_entry.obs/symlink_status.pass.cpp
index 24e806950952..77da936382aa 100644
--- a/libcxx/test/std/input.output/filesystems/class.directory_entry/directory_entry.obs/symlink_status.pass.cpp
+++ b/libcxx/test/std/input.output/filesystems/class.directory_entry/directory_entry.obs/symlink_status.pass.cpp
@@ -44,7 +44,7 @@ static void test_signature() {
     file_status es = e.symlink_status(eec);
     assert(ps.type() == es.type());
     assert(ps.permissions() == es.permissions());
-    assert(pec == eec);
+    assert(pec.default_error_condition() == eec.default_error_condition());
   }
   for (const auto& p : TestCases) {
     const directory_entry e(p);
diff --git a/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.copy_file/copy_file_procfs.pass.cpp b/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.copy_file/copy_file_procfs.pass.cpp
new file mode 100644
index 000000000000..29bc8e41250d
--- /dev/null
+++ b/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.copy_file/copy_file_procfs.pass.cpp
@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14
+// REQUIRES: linux
+// UNSUPPORTED: no-filesystem
+// XFAIL: no-localization
+// UNSUPPORTED: availability-filesystem-missing
+
+// <filesystem>
+
+// bool copy_file(const path& from, const path& to);
+// bool copy_file(const path& from, const path& to, error_code& ec) noexcept;
+// bool copy_file(const path& from, const path& to, copy_options options);
+// bool copy_file(const path& from, const path& to, copy_options options,
+//           error_code& ec) noexcept;
+
+#include <cassert>
+#include <filesystem>
+#include <system_error>
+
+#include "test_macros.h"
+#include "filesystem_test_helper.h"
+
+namespace fs = std::filesystem;
+
+// Linux has various virtual filesystems such as /proc and /sys
+// where files may have no length (st_size == 0), but still contain data.
+// This is because the to-be-read data is usually generated ad-hoc by the reading syscall
+// These files can not be copied with kernel-side copies like copy_file_range or sendfile,
+// and must instead be copied via a traditional userspace read + write loop.
+int main(int, char** argv) {
+  const fs::path procfile{"/proc/self/comm"};
+  assert(file_size(procfile) == 0);
+
+  scoped_test_env env;
+  std::error_code ec = GetTestEC();
+
+  const fs::path dest = env.make_env_path("dest");
+
+  assert(copy_file(procfile, dest, ec));
+  assert(!ec);
+
+  // /proc/self/comm contains the filename of the executable, plus a null terminator
+  assert(file_size(dest) == fs::path(argv[0]).filename().string().size() + 1);
+
+  return 0;
+}
diff --git a/libcxx/test/support/filesystem_test_helper.h b/libcxx/test/support/filesystem_test_helper.h
index a63d645d1a01..2ad9efb32c60 100644
--- a/libcxx/test/support/filesystem_test_helper.h
+++ b/libcxx/test/support/filesystem_test_helper.h
@@ -583,7 +583,11 @@ struct ExceptionChecker {
     assert(ErrorIsImp(Err.code(), {expected_err}));
     assert(Err.path1() == expected_path1);
     assert(Err.path2() == expected_path2);
+#ifndef _WIN32
+    // On Windows, the error strings are windows error code strings, and don't
+    // match textually with the strings generated for generic std::errc::*.
     LIBCPP_ONLY(check_libcxx_string(Err));
+#endif
   }
 
   void check_libcxx_string(fs::filesystem_error const& Err) {
diff --git a/libcxx/test/tools/clang_tidy_checks/CMakeLists.txt b/libcxx/test/tools/clang_tidy_checks/CMakeLists.txt
index 05c44e49b448..0f8f0e8864d0 100644
--- a/libcxx/test/tools/clang_tidy_checks/CMakeLists.txt
+++ b/libcxx/test/tools/clang_tidy_checks/CMakeLists.txt
@@ -92,6 +92,7 @@ set(SOURCES
     header_exportable_declarations.cpp
     hide_from_abi.cpp
     internal_ftm_use.cpp
+    nodebug_on_aliases.cpp
     proper_version_checks.cpp
     qualify_declval.cpp
     robust_against_adl.cpp
diff --git a/libcxx/test/tools/clang_tidy_checks/libcpp_module.cpp b/libcxx/test/tools/clang_tidy_checks/libcpp_module.cpp
index 54beed5e30be..bc7c8ce7ec44 100644
--- a/libcxx/test/tools/clang_tidy_checks/libcpp_module.cpp
+++ b/libcxx/test/tools/clang_tidy_checks/libcpp_module.cpp
@@ -13,6 +13,7 @@
 #include "header_exportable_declarations.hpp"
 #include "hide_from_abi.hpp"
 #include "internal_ftm_use.hpp"
+#include "nodebug_on_aliases.hpp"
 #include "proper_version_checks.hpp"
 #include "qualify_declval.hpp"
 #include "robust_against_adl.hpp"
@@ -26,6 +27,7 @@ public:
     check_factories.registerCheck<libcpp::header_exportable_declarations>("libcpp-header-exportable-declarations");
     check_factories.registerCheck<libcpp::hide_from_abi>("libcpp-hide-from-abi");
     check_factories.registerCheck<libcpp::internal_ftm_use>("libcpp-internal-ftms");
+    check_factories.registerCheck<libcpp::nodebug_on_aliases>("libcpp-nodebug-on-aliases");
     check_factories.registerCheck<libcpp::proper_version_checks>("libcpp-cpp-version-check");
     check_factories.registerCheck<libcpp::robust_against_adl_check>("libcpp-robust-against-adl");
     check_factories.registerCheck<libcpp::uglify_attributes>("libcpp-uglify-attributes");
diff --git a/libcxx/test/tools/clang_tidy_checks/nodebug_on_aliases.cpp b/libcxx/test/tools/clang_tidy_checks/nodebug_on_aliases.cpp
new file mode 100644
index 000000000000..9b96269e5980
--- /dev/null
+++ b/libcxx/test/tools/clang_tidy_checks/nodebug_on_aliases.cpp
@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang-tidy/ClangTidyCheck.h"
+
+#include "nodebug_on_aliases.hpp"
+#include "utilities.hpp"
+
+namespace libcpp {
+namespace {
+AST_MATCHER(clang::NamedDecl, isPretty) { return !is_ugly_name(Node.getName()); }
+} // namespace
+
+nodebug_on_aliases::nodebug_on_aliases(llvm::StringRef name, clang::tidy::ClangTidyContext* context)
+    : clang::tidy::ClangTidyCheck(name, context) {}
+
+void nodebug_on_aliases::registerMatchers(clang::ast_matchers::MatchFinder* finder) {
+  using namespace clang::ast_matchers;
+  finder->addMatcher(
+      typeAliasDecl(unless(anyOf(isPretty(), hasAttr(clang::attr::NoDebug), hasAncestor(functionDecl()))))
+          .bind("nodebug_on_internal_aliases"),
+      this);
+}
+
+void nodebug_on_aliases::check(const clang::ast_matchers::MatchFinder::MatchResult& result) {
+  if (const auto* alias = result.Nodes.getNodeAs<clang::TypeAliasDecl>("nodebug_on_internal_aliases")) {
+    diag(alias->getBeginLoc(), "Internal aliases should always be marked _LIBCPP_NODEBUG");
+  }
+}
+} // namespace libcpp
diff --git a/libcxx/test/tools/clang_tidy_checks/nodebug_on_aliases.hpp b/libcxx/test/tools/clang_tidy_checks/nodebug_on_aliases.hpp
new file mode 100644
index 000000000000..1097e8910649
--- /dev/null
+++ b/libcxx/test/tools/clang_tidy_checks/nodebug_on_aliases.hpp
@@ -0,0 +1,18 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang-tidy/ClangTidyCheck.h"
+
+namespace libcpp {
+class nodebug_on_aliases : public clang::tidy::ClangTidyCheck {
+public:
+  nodebug_on_aliases(llvm::StringRef, clang::tidy::ClangTidyContext*);
+  void registerMatchers(clang::ast_matchers::MatchFinder*) override;
+  void check(const clang::ast_matchers::MatchFinder::MatchResult&) override;
+};
+} // namespace libcpp
diff --git a/libcxx/test/tools/clang_tidy_checks/uglify_attributes.cpp b/libcxx/test/tools/clang_tidy_checks/uglify_attributes.cpp
index 7812b236f613..24bacde6304b 100644
--- a/libcxx/test/tools/clang_tidy_checks/uglify_attributes.cpp
+++ b/libcxx/test/tools/clang_tidy_checks/uglify_attributes.cpp
@@ -10,20 +10,11 @@
 #include "clang-tidy/ClangTidyModuleRegistry.h"
 
 #include "uglify_attributes.hpp"
+#include "utilities.hpp"
 
-#include <algorithm>
-#include <array>
-#include <span>
-#include <string_view>
+#include <optional>
 
 namespace {
-bool isUgly(std::string_view str) {
-  if (str.size() < 2)
-    return false;
-  if (str[0] == '_' && str[1] >= 'A' && str[1] <= 'Z')
-    return true;
-  return str.find("__") != std::string_view::npos;
-}
 
 // Starting with Clang 17 ToT C++23 support is provided by CPlusPlus23 instead
 // of C++23 support is provided by CPlusPlus2b. To allow a smooth transition for
@@ -77,17 +68,15 @@ AST_MATCHER(clang::Attr, isPretty) {
   if (Node.isKeywordAttribute() || !Node.getAttrName())
     return false;
   if (Node.isCXX11Attribute() && !Node.hasScope()) {
-    if (isUgly(Node.getAttrName()->getName()))
+    if (is_ugly_name(Node.getAttrName()->getName()))
       return false;
     return !llvm::is_contained(
         get_standard_attributes(Finder->getASTContext().getLangOpts()), Node.getAttrName()->getName());
   }
   if (Node.hasScope())
-    if (!isUgly(Node.getScopeName()->getName()))
+    if (!is_ugly_name(Node.getScopeName()->getName()))
       return true;
-  return !isUgly(Node.getAttrName()->getName());
-
-  return false;
+  return !is_ugly_name(Node.getAttrName()->getName());
 }
 
 std::optional<std::string> getUglyfiedCXX11Attr(const clang::Attr& attr) {
diff --git a/libcxx/test/tools/clang_tidy_checks/utilities.hpp b/libcxx/test/tools/clang_tidy_checks/utilities.hpp
new file mode 100644
index 000000000000..b780efef2385
--- /dev/null
+++ b/libcxx/test/tools/clang_tidy_checks/utilities.hpp
@@ -0,0 +1,22 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LIBCXX_TEST_TOOLS_CLANG_TIDY_CHECKS_UTILITIES_HPP
+#define LIBCXX_TEST_TOOLS_CLANG_TIDY_CHECKS_UTILITIES_HPP
+
+#include <string_view>
+
+inline bool is_ugly_name(std::string_view str) {
+  if (str.size() < 2)
+    return false;
+  if (str[0] == '_' && str[1] >= 'A' && str[1] <= 'Z')
+    return true;
+  return str.find("__") != std::string_view::npos;
+}
+
+#endif // LIBCXX_TEST_TOOLS_CLANG_TIDY_CHECKS_UTILITIES_HPP
diff --git a/libcxx/utils/libcxx-benchmark-json b/libcxx/utils/libcxx-benchmark-json
new file mode 100755
index 000000000000..7f743c32caf4
--- /dev/null
+++ b/libcxx/utils/libcxx-benchmark-json
@@ -0,0 +1,57 @@
+#!/usr/bin/env bash
+
+set -e
+
+PROGNAME="$(basename "${0}")"
+MONOREPO_ROOT="$(realpath $(dirname "${PROGNAME}"))"
+function usage() {
+cat <<EOF
+Usage:
+${PROGNAME} [-h|--help] <build-directory> benchmarks...
+
+Print the path to the JSON files containing benchmark results for the given benchmarks.
+
+This requires those benchmarks to have already been run, i.e. this only resolves the path
+to the benchmark .json file within the build directory.
+
+<build-directory>  The path to the build directory.
+benchmarks...      Paths of the benchmarks to extract the results for. Those paths are relative to '<monorepo-root>'.
+
+Example
+=======
+$ cmake -S runtimes -B build/ -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi"
+$ libcxx-lit build/ -sv libcxx/test/benchmarks/algorithms/for_each.bench.cpp
+$ less \$(${PROGNAME} build/ libcxx/test/benchmarks/algorithms/for_each.bench.cpp)
+EOF
+}
+
+if [[ "${1}" == "-h" || "${1}" == "--help" ]]; then
+    usage
+    exit 0
+fi
+
+if [[ $# -lt 1 ]]; then
+    usage
+    exit 1
+fi
+
+build_dir="${1}"
+shift
+
+for benchmark in ${@}; do
+    # Normalize the paths by turning all benchmarks paths into absolute ones and then making them
+    # relative to the root of the monorepo.
+    benchmark="$(realpath ${benchmark})"
+    relative=$(python -c "import os; import sys; print(os.path.relpath(sys.argv[1], sys.argv[2]))" "${benchmark}" "${MONOREPO_ROOT}")
+
+    # Extract components of the benchmark path
+    directory="$(dirname ${relative})"
+    file="$(basename ${relative})"
+
+    # Reconstruct the (slightly weird) path to the benchmark json file. This should be kept in sync
+    # whenever the test suite changes.
+    json="${build_dir}/${directory}/Output/${file}.dir/benchmark-result.json"
+    if [[ -f "${json}" ]]; then
+        echo "${json}"
+    fi
+done
diff --git a/libcxx/utils/libcxx-compare-benchmarks b/libcxx/utils/libcxx-compare-benchmarks
new file mode 100755
index 000000000000..e04820fc57ed
--- /dev/null
+++ b/libcxx/utils/libcxx-compare-benchmarks
@@ -0,0 +1,62 @@
+#!/usr/bin/env bash
+
+set -e
+
+PROGNAME="$(basename "${0}")"
+MONOREPO_ROOT="$(realpath $(dirname "${PROGNAME}"))"
+function usage() {
+cat <<EOF
+Usage:
+${PROGNAME} [-h|--help] <baseline-build> <candidate-build> benchmarks...
+
+Compare the given benchmarks between the baseline and the candidate build directories.
+
+This requires those benchmarks to have already been generated in both build directories.
+
+<baseline-build>   The path to the build directory considered the baseline.
+<candidate-build>  The path to the build directory considered the candidate.
+benchmarks...      Paths of the benchmarks to compare. Those paths are relative to '<monorepo-root>'.
+
+Example
+=======
+$ libcxx-lit build1/ -sv libcxx/test/benchmarks/algorithms/for_each.bench.cpp
+$ libcxx-lit build2/ -sv libcxx/test/benchmarks/algorithms/for_each.bench.cpp
+$ ${PROGNAME} build1/ build2/ libcxx/test/benchmarks/algorithms/for_each.bench.cpp
+EOF
+}
+
+if [[ "${1}" == "-h" || "${1}" == "--help" ]]; then
+    usage
+    exit 0
+fi
+
+if [[ $# -lt 1 ]]; then
+    usage
+    exit 1
+fi
+
+baseline="${1}"
+candidate="${2}"
+shift; shift
+
+GBENCH="${MONOREPO_ROOT}/third-party/benchmark"
+
+python3 -m venv /tmp/libcxx-compare-benchmarks-venv
+source /tmp/libcxx-compare-benchmarks-venv/bin/activate
+pip3 install -r ${GBENCH}/tools/requirements.txt
+
+for benchmark in ${@}; do
+    base="$(${MONOREPO_ROOT}/libcxx/utils/libcxx-benchmark-json ${baseline} ${benchmark})"
+    cand="$(${MONOREPO_ROOT}/libcxx/utils/libcxx-benchmark-json ${candidate} ${benchmark})"
+
+    if [[ ! -e "${base}" ]]; then
+        echo "Benchmark ${benchmark} does not exist in the baseline"
+        continue
+    fi
+    if [[ ! -e "${cand}" ]]; then
+        echo "Benchmark ${benchmark} does not exist in the candidate"
+        continue
+    fi
+
+    "${GBENCH}/tools/compare.py" benchmarks "${base}" "${cand}"
+done