[libc++] Fix UB in bitwise logic of {std, ranges}::{fill, fill_n} algorithms (#122410)

This PR addresses an undefined behavior that arises when using the
`std::fill` and `std::fill_n` algorithms, as well as their ranges
counterparts `ranges::fill` and `ranges::fill_n`, with `vector<bool, Alloc>`
that utilizes a custom-sized allocator with small integral types.

1 files changed, 2 insertions, 10 deletions

diff --git a/libcxx/include/__algorithm/fill_n.h b/libcxx/include/__algorithm/fill_n.h
index a7e01c45b922..0da78e1f38c4 100644
--- a/libcxx/include/__algorithm/fill_n.h
+++ b/libcxx/include/__algorithm/fill_n.h
@@ -41,11 +41,7 @@ __fill_n_bool(__bit_iterator<_Cp, false> __first, typename __size_difference_typ
   if (__first.__ctz_ != 0) {
     __storage_type __clz_f = static_cast<__storage_type>(__bits_per_word - __first.__ctz_);
     __storage_type __dn    = std::min(__clz_f, __n);
-    __storage_type __m     = (~__storage_type(0) << __first.__ctz_) & (~__storage_type(0) >> (__clz_f - __dn));
-    if (_FillVal)
-      *__first.__seg_ |= __m;
-    else
-      *__first.__seg_ &= ~__m;
+    std::__fill_masked_range(std::__to_address(__first.__seg_), __clz_f - __dn, __first.__ctz_, _FillVal);
     __n -= __dn;
     ++__first.__seg_;
   }
@@ -56,11 +52,7 @@ __fill_n_bool(__bit_iterator<_Cp, false> __first, typename __size_difference_typ
   // do last partial word
   if (__n > 0) {
     __first.__seg_ += __nw;
-    __storage_type __m = ~__storage_type(0) >> (__bits_per_word - __n);
-    if (_FillVal)
-      *__first.__seg_ |= __m;
-    else
-      *__first.__seg_ &= ~__m;
+    std::__fill_masked_range(std::__to_address(__first.__seg_), __bits_per_word - __n, 0u, _FillVal);
   }
 }