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_        = __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_ = __fill_range_in_word(*__first.__seg_, 0u, __bits_per_word - __n, _FillVal);
   }
 }
 

libcxx/include/__fwd/bit_reference.h

@@ -24,6 +24,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Cp, bool _IsConst, typename _Cp::__storage_type = 0>
 class __bit_iterator;
 
+template <class, class = void>
+struct __size_difference_type_traits;
+
 template <class _StorageType, __enable_if_t<is_unsigned<_StorageType>::value, int> = 0>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _StorageType
 __fill_range_in_word(_StorageType __word, unsigned __ctz, unsigned __clz, bool __fill_val) {

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