This adjusts the return statements of optional::value_or and
expected::value_or to not perform explicit conversions, so that the
actual conversion performed matches the requirements expressed in the
Mandates: elements (LWG 4406).

Also adjust the return types to remove cv-qualifiers (LWG 3424).

libstdc++-v3/ChangeLog:

	* include/std/expected (expected::value_or): Use remove_cv_t for
	the return type. Do not use static_cast for return statement.
	Adjust static_assert conditions to match return statements.
	* include/std/optional (optional::value_or): Likewise.
	(optional<T&>::value_or): Likewise.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

libstdc++-v3/ChangeLog:

	* include/std/istream (ignore): Add an overload for char.
	* testsuite/27_io/basic_istream/ignore/char/93672.cc: Adjust
	expected behaviour for C++26 mode.
	* testsuite/27_io/basic_istream/ignore/char/4.cc: New test.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

When using directly __gnu_debug::vector the std::erase_if is called with a
reference to the std::vector base class and so is missing the invalidation
of the iterators implied by this operation.

To fix this provide a std::erase_if overload dedicated to __gnu_debug::vector.
Doing so we can cleanup the implementation dedicated to std::vector from any
_GLIBCXX_DEBUG consideration.

libstdc++-v3/ChangeLog:

	* include/debug/vector (std::erase_if, std::erase): New overloads for
	std::__debug::vector instances.
	* include/std/vector (std::erase_if, std::erase): Make overloads specific
	to normal std::vector implementation.
	* testsuite/23_containers/vector/debug/erase.cc: New test case.
	* testsuite/23_containers/vector/debug/invalidation/erase.cc: New test case.

The changes needed for submdspan are:

  * In submdspan related code the user-defined integer-like
    types need to be copy- and move-constructable.

  * The traits for writing tests that work with both left- and right,
    possibly padded, layouts will also be useful for submdspan.
    Therefore, this code is moved up and generalized.

  * Move __offset further up in <mdspan> and fix some formatting
    mistakes.

libstdc++-v3/ChangeLog:

	* include/std/mdspan: Improve formatting and placement.
	* testsuite/23_containers/mdspan/int_like.h: Optionally,
	add move- and copy-ctors.
	* testsuite/23_containers/mdspan/layouts/padded_traits.h: Move to...
	* testsuite/23_containers/mdspan/layout_traits.h: ...here.
	* testsuite/23_containers/mdspan/layouts/ctors.cc: Fix include.
	* testsuite/23_containers/mdspan/layouts/mapping.cc: Ditto.
	* testsuite/23_containers/mdspan/layouts/padded.cc: Ditto.
	* testsuite/23_containers/mdspan/layouts/padded_neg.cc: Ditto.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>

These three changes are needed to make <mdspan> compatible with Clang:

  - the type alias _Storage must occur before its first use.

  - the friend declarations of function must match exactly, including
  noexcept and constexpr.

  - the 'template' in typename T::template type<double>.

libstdc++-v3/ChangeLog:

	* include/std/mdspan (extents::_Storage): Move type alias before
	its first use.
	(__mdspan::__static_extents): Add missing noexcept and constexpr
	to friend declaration in extents.
	(__mdspan::__dynamic_extents): Ditto.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>

The following patch attempts to implement CWG1670 and related LWG4468.

2025-11-19  Jakub Jelinek  <jakub@redhat.com>

gcc/cp/
	* parser.cc (cp_parser_conversion_type_id): Implement C++ DR1670
	- auto as conversion-type-id.  Pedwarn on conversion operators
	with placeholder return type.
gcc/testsuite/
	* g++.dg/DRs/dr1670-1.C: New test.
	* g++.dg/DRs/dr1670-2.C: New test.
	* g++.dg/DRs/dr1670-3.C: New test.
	* g++.dg/modules/auto-1_a.H: Use dg-options instead of
	dg-additional-options.
	* g++.dg/modules/auto-1_b.C: Likewise.
	* g++.dg/cpp1y/auto-fn12.C: Likewise.
	* g++.dg/cpp1y/auto-fn13.C: Add empty dg-options.
	* g++.dg/cpp1y/auto-fn22.C: Likewise.
	* g++.dg/cpp1y/constexpr-assert2.C: Likewise.
	* g++.dg/cpp1y/auto-fn44.C: Add dg-options -Wpedantic and expect
	further warnings.
	* g++.dg/cpp1y/auto-fn50.C: Likewise.
	* g++.dg/cpp0x/auto9.C: Expect two errors always rather than just
	for C++11.
libstdc++-v3/
	* include/std/type_traits (constant_wrapper conversion operator):
	Use decltype(value) instead of decltype(auto).  Resolves LWG4468.

Make the shared_mutex::try_lock(), shared_timed_mutex::try_lock_until()
and shared_timed_mutex::try_lock_shared_until() all handle errors from
pthread functions consistently by returning false to indicate that the
lock could not be taken. If _GLIBCXX_ASSERTIONS is defined then
unexpected errors, such as EDEADLK and EINVAL will cause an assertion
failure. If _GLIBCXX_ASSERTIONS is not defined then these functions no
longer ever return true incorrectly indicating that they have taken the
lock.

This removes the previous behaviour of looping on EDEADLK in
try_lock_shared_until() and no longer returns true on EINVAL in all of
these functions. (In theory at least it should not be possible to
trigger EINVAL since 5dba17a3e709859968f939354e6e5e8d796012d3.)

Unfortunately my reading of POSIX is that pthread_rwlock_clockrdlock[1],
pthread_rwlock_timedrdlock pthread_rwlock_clockwrlock[2] and
pthread_rwlock_timedwrlock are allowed to deadlock rather than return
EDEADLK when trying to take a lock a second time from the same
thread. This means that the deadlock tests cannot be enabled by
default. I believe that the tests do work with glibc (2.31 & 2.36) and
with the __shared_mutex_cv implementation though.

[1] https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_rwlock_clockrdlock.html
[2] https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_rwlock_clockwrlock.html

libstdc++-v3/ChangeLog:

	* include/std/shared_mutex (try_lock, try_lock_until)
	(try_lock_shared_until): Respond consistently to errors and
	deadlocks.
	* testsuite/30_threads/shared_timed_mutex/try_lock_until/116586.cc:
	Test deadlock behaviour if possible.

Signed-off-by: Mike Crowe <mac@mcrowe.com>

Previously, _Utf_view accepted any input_range, including reference-to-array
types like char(&)[2], and stored it as the _M_base member. In such cases,
_Utf_view was not assignable, failing the requirements of view concept.

This patch addresses the issue by adding the ranges::view constraint to the
second template parameter of _Utf_view, and for clarity renaming it from
_Range to _View. The constructor is also adjusted to accept its argument
by value (views must be O(1) move-constructible). This prevents implicitly
generated CTAD from deducing a reference type.

This makes _Utf_view consistent with both other standard views and the
wording from P2728R8: Unicode in the Library, Part 1: UTF Transcoding [1].

The explicit CTAD from viewable_range is not defined for _Utf_view because
it depends on views::all_t, views::ref_view, and views::owning_view,
which are declared in <ranges>. Consequently, users must explicitly cast
the argument to a view or specify it as a template parameter.

[1] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p2728r8.html

libstdc++-v3/ChangeLog:

	* include/bits/unicode.h (_Utf_view): Rename the template parameter
	from _Range to _View and constrain it with ranges::view.
	(_Utf_view::_Utf_view): Accept by value instead of rvalue reference.
	* include/std/format (__format::__write_padded): Replace _Utf_view
	over const char32_t(&)[1] with span<const char32_t, 1>.
	* testsuite/ext/unicode/view.cc: Add checks if specialization
	of _Utf_view satisfy view. Wrap arrays into std::span before
	constructing _Utf_view.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

libstdc++-v3/ChangeLog:

	* include/std/bitset: Add comment to feature test macro test.

According to the standard the first n characters of a bitset constructor
string need to be checked instead of only N.

libstdc++-v3/ChangeLog:

	PR libstdc++/121054
	* include/std/bitset: Add string check to constructor.
	* testsuite/20_util/bitset/121054.cc: New test.
	* testsuite/20_util/bitset/cons/constexpr_c++23.cc: Fix.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>