[libc++] Support constexpr for std::stable_sort in radix sort branch (#125284)

`std::stable_sort` is `constexpr` since PR
#110320
But `radix_sort` branch is still non-`constexpr`.
This PR fixes it.

#119394
#105360

Author: izvolov

libcxx/include/__algorithm/radix_sort.h

@@ -33,6 +33,7 @@
 #include <__bit/countl.h>
 #include <__config>
 #include <__functional/identity.h>
+#include <__iterator/access.h>
 #include <__iterator/distance.h>
 #include <__iterator/iterator_traits.h>
 #include <__iterator/move_iterator.h>
@@ -67,7 +68,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #if _LIBCPP_STD_VER >= 14
 
 template <class _InputIterator, class _OutputIterator>
-_LIBCPP_HIDE_FROM_ABI pair<_OutputIterator, __iter_value_type<_InputIterator>>
+_LIBCPP_HIDE_FROM_ABI constexpr pair<_OutputIterator, __iter_value_type<_InputIterator>>
 __partial_sum_max(_InputIterator __first, _InputIterator __last, _OutputIterator __result) {
   if (__first == __last)
     return {__result, 0};
@@ -109,15 +110,15 @@ struct __counting_sort_traits {
 };
 
 template <class _Radix, class _Integer>
-_LIBCPP_HIDE_FROM_ABI auto __nth_radix(size_t __radix_number, _Radix __radix, _Integer __n) {
+_LIBCPP_HIDE_FROM_ABI constexpr auto __nth_radix(size_t __radix_number, _Radix __radix, _Integer __n) {
   static_assert(is_unsigned<_Integer>::value);
   using __traits = __counting_sort_traits<_Integer, _Radix>;
 
   return __radix(static_cast<_Integer>(__n >> __traits::__radix_size * __radix_number));
 }
 
 template <class _ForwardIterator, class _Map, class _RandomAccessIterator>
-_LIBCPP_HIDE_FROM_ABI void
+_LIBCPP_HIDE_FROM_ABI constexpr void
 __collect(_ForwardIterator __first, _ForwardIterator __last, _Map __map, _RandomAccessIterator __counters) {
   using __value_type = __iter_value_type<_ForwardIterator>;
   using __traits     = __counting_sort_traits<__value_type, _Map>;
@@ -129,7 +130,7 @@ __collect(_ForwardIterator __first, _ForwardIterator __last, _Map __map, _Random
 }
 
 template <class _ForwardIterator, class _RandomAccessIterator1, class _Map, class _RandomAccessIterator2>
-_LIBCPP_HIDE_FROM_ABI void
+_LIBCPP_HIDE_FROM_ABI constexpr void
 __dispose(_ForwardIterator __first,
           _ForwardIterator __last,
           _RandomAccessIterator1 __result,
@@ -147,7 +148,7 @@ template <class _ForwardIterator,
           class _RandomAccessIterator1,
           class _RandomAccessIterator2,
           size_t... _Radices>
-_LIBCPP_HIDE_FROM_ABI bool __collect_impl(
+_LIBCPP_HIDE_FROM_ABI constexpr bool __collect_impl(
     _ForwardIterator __first,
     _ForwardIterator __last,
     _Map __map,
@@ -177,7 +178,7 @@ _LIBCPP_HIDE_FROM_ABI bool __collect_impl(
 }
 
 template <class _ForwardIterator, class _Map, class _Radix, class _RandomAccessIterator1, class _RandomAccessIterator2>
-_LIBCPP_HIDE_FROM_ABI bool
+_LIBCPP_HIDE_FROM_ABI constexpr bool
 __collect(_ForwardIterator __first,
           _ForwardIterator __last,
           _Map __map,
@@ -191,7 +192,7 @@ __collect(_ForwardIterator __first,
 }
 
 template <class _BidirectionalIterator, class _RandomAccessIterator1, class _Map, class _RandomAccessIterator2>
-_LIBCPP_HIDE_FROM_ABI void __dispose_backward(
+_LIBCPP_HIDE_FROM_ABI constexpr void __dispose_backward(
     _BidirectionalIterator __first,
     _BidirectionalIterator __last,
     _RandomAccessIterator1 __result,
@@ -206,7 +207,7 @@ _LIBCPP_HIDE_FROM_ABI void __dispose_backward(
 }
 
 template <class _ForwardIterator, class _RandomAccessIterator, class _Map>
-_LIBCPP_HIDE_FROM_ABI _RandomAccessIterator
+_LIBCPP_HIDE_FROM_ABI constexpr _RandomAccessIterator
 __counting_sort_impl(_ForwardIterator __first, _ForwardIterator __last, _RandomAccessIterator __result, _Map __map) {
   using __value_type = __iter_value_type<_ForwardIterator>;
   using __traits     = __counting_sort_traits<__value_type, _Map>;
@@ -225,7 +226,7 @@ template <class _RandomAccessIterator1,
           class _Radix,
           enable_if_t< __radix_sort_traits<__iter_value_type<_RandomAccessIterator1>, _Map, _Radix>::__radix_count == 1,
                        int> = 0>
-_LIBCPP_HIDE_FROM_ABI void __radix_sort_impl(
+_LIBCPP_HIDE_FROM_ABI constexpr void __radix_sort_impl(
     _RandomAccessIterator1 __first,
     _RandomAccessIterator1 __last,
     _RandomAccessIterator2 __buffer,
@@ -245,7 +246,7 @@ template <
     class _Radix,
     enable_if_t< __radix_sort_traits<__iter_value_type<_RandomAccessIterator1>, _Map, _Radix>::__radix_count % 2 == 0,
                  int> = 0 >
-_LIBCPP_HIDE_FROM_ABI void __radix_sort_impl(
+_LIBCPP_HIDE_FROM_ABI constexpr void __radix_sort_impl(
     _RandomAccessIterator1 __first,
     _RandomAccessIterator1 __last,
     _RandomAccessIterator2 __buffer_begin,
@@ -307,7 +308,7 @@ struct __low_byte_fn {
 };
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _Map, class _Radix>
-_LIBCPP_HIDE_FROM_ABI void
+_LIBCPP_HIDE_FROM_ABI constexpr void
 __radix_sort(_RandomAccessIterator1 __first,
              _RandomAccessIterator1 __last,
              _RandomAccessIterator2 __buffer,
@@ -318,7 +319,7 @@ __radix_sort(_RandomAccessIterator1 __first,
 }
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2>
-_LIBCPP_HIDE_FROM_ABI void
+_LIBCPP_HIDE_FROM_ABI constexpr void
 __radix_sort(_RandomAccessIterator1 __first, _RandomAccessIterator1 __last, _RandomAccessIterator2 __buffer) {
   std::__radix_sort(__first, __last, __buffer, __identity{}, __low_byte_fn{});
 }

libcxx/include/__algorithm/stable_sort.h

@@ -25,6 +25,7 @@
 #include <__memory/unique_temporary_buffer.h>
 #include <__type_traits/desugars_to.h>
 #include <__type_traits/enable_if.h>
+#include <__type_traits/is_constant_evaluated.h>
 #include <__type_traits/is_integral.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_trivially_assignable.h>
@@ -253,6 +254,12 @@ _LIBCPP_CONSTEXPR_SINCE_CXX26 void __stable_sort(
   if constexpr (__allowed_radix_sort) {
     if (__len <= __buff_size && __len >= static_cast<difference_type>(__radix_sort_min_bound<value_type>()) &&
         __len <= static_cast<difference_type>(__radix_sort_max_bound<value_type>())) {
+      if (__libcpp_is_constant_evaluated()) {
+        for (auto* __p = __buff; __p < __buff + __buff_size; ++__p) {
+          std::__construct_at(__p);
+        }
+      }
+
       std::__radix_sort(__first, __last, __buff);
       return;
     }

libcxx/test/std/algorithms/alg.sorting/alg.sort/stable.sort/stable_sort.pass.cpp

@@ -199,6 +199,8 @@ int main(int, char**) {
 #if TEST_STD_VER >= 26
   static_assert(test<int>());
   static_assert(test<float>());
+  // test constexprness of radix sort branch
+  static_assert(test<char>());
 #endif
   return 0;
 }