Address @ldionne's review comments

Author: winner245

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     = std::__middle_mask<__storage_type>(__first.__ctz_, __clz_f - __dn);
-    if (_FillVal)
-      *__first.__seg_ |= __m;
-    else
-      *__first.__seg_ &= ~__m;
+    *__first.__seg_        = std::__fill_range_in_word(*__first.__seg_, __first.__ctz_, __clz_f - __dn, _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 = std::__trailing_mask<__storage_type>(__bits_per_word - __n);
-    if (_FillVal)
-      *__first.__seg_ |= __m;
-    else
-      *__first.__seg_ &= ~__m;
+    *__first.__seg_ = std::__fill_range_in_word(*__first.__seg_, 0u, __bits_per_word - __n, _FillVal);
   }
 }
 

libcxx/include/__fwd/bit_reference.h

@@ -12,6 +12,7 @@
 #include <__config>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_unsigned.h>
+#include <climits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -25,20 +26,18 @@ class __bit_iterator;
 template <class, class = void>
 struct __size_difference_type_traits;
 
+// This function is designed to operate correctly even for smaller integral types like `uint8_t`, `uint16_t`,
+// or `unsigned short`. Casting back to _StorageType is crucial to prevent undefined behavior that can arise
+// from integral promotions.
+// See https://github.com/llvm/llvm-project/pull/122410
 template <class _StorageType, __enable_if_t<is_unsigned<_StorageType>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _StorageType __leading_mask(unsigned __shift) {
-  return static_cast<_StorageType>(static_cast<_StorageType>(~static_cast<_StorageType>(0)) << __shift);
-}
-
-template <class _StorageType, __enable_if_t<is_unsigned<_StorageType>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _StorageType __trailing_mask(unsigned __shift) {
-  return static_cast<_StorageType>(static_cast<_StorageType>(~static_cast<_StorageType>(0)) >> __shift);
-}
-
-template <class _StorageType, __enable_if_t<is_unsigned<_StorageType>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _StorageType __middle_mask(unsigned __lshift, unsigned __rshift) {
-  return static_cast<_StorageType>(
-      std::__leading_mask<_StorageType>(__lshift) & std::__trailing_mask<_StorageType>(__rshift));
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _StorageType
+__fill_range_in_word(_StorageType __word, unsigned __ctz, unsigned __clz, bool __fill_val) {
+  _LIBCPP_ASSERT_VALID_INPUT_RANGE(
+      __ctz + __clz < sizeof(_StorageType) * CHAR_BIT, "__fill_range called with invalid range");
+  _StorageType __m = static_cast<_StorageType>(static_cast<_StorageType>(~static_cast<_StorageType>(0)) << __ctz) &
+                     static_cast<_StorageType>(static_cast<_StorageType>(~static_cast<_StorageType>(0)) >> __clz);
+  return __fill_val ? __word | __m : __word & ~__m;
 }
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/test/support/sized_allocator.h

@@ -16,15 +16,15 @@
 
 #include "test_macros.h"
 
-template <typename T, typename SIZE_TYPE = std::size_t, typename DIFF_TYPE = std::ptrdiff_t>
+template <typename T, typename Size = std::size_t, typename Difference = std::ptrdiff_t>
 class sized_allocator {
   template <typename U, typename Sz, typename Diff>
   friend class sized_allocator;
 
 public:
   using value_type                  = T;
-  using size_type                   = SIZE_TYPE;
-  using difference_type             = DIFF_TYPE;
+  using size_type                   = Size;
+  using difference_type             = Difference;
   using propagate_on_container_swap = std::true_type;
 
   TEST_CONSTEXPR_CXX20 explicit sized_allocator(int d = 0) : data_(d) {}
@@ -55,4 +55,4 @@ class sized_allocator {
   }
 };
 
-#endif
+#endif // TEST_SUPPORT_SIZED_ALLOCATOR_H

libcxx/utils/libcxx/test/params.py

@@ -31,6 +31,7 @@
     "-Wno-reserved-module-identifier",
     '-Wdeprecated-copy',
     '-Wdeprecated-copy-dtor',
+    "-Wshift-negative-value",
     # GCC warns about places where we might want to add sized allocation/deallocation
     # functions, but we know better what we're doing/testing in the test suite.
     "-Wno-sized-deallocation",