[PATCH] libstdc++: Use feature test macros in <bits/stl_construct.h>

[PATCH] libstdc++: Use feature test macros in <bits/stl_construct.h>

[Jonathan Wakely]

Does anybody see any issues with making this change?

The __cpp_constexpr_dynamic_alloc macro is defined with -std=c++2a
since Clang 10.0.0 so this won't result in std::construct_at
disappearing by anybody using libstdc++ with Clang.

Tested x86_64-linux and aarch64-linux. Basic smoke tests checked with
Clang 16 too.

-- >8 --

The preprocessor checks for __cplusplus in <bits/stl_construct.h> should
use the appropriate feature test macros instead of __cplusplus, namely
__glibcxx_raw_memory_algorithms and __cpp_constexpr_dynamic_alloc.

For the latter, we want to check the compiler macro not the library's
__cpp_lib_constexpr_dynamic_alloc, because the latter is not defined for
freestanding but std::construct_at needs to be.

libstdc++-v3/ChangeLog:

	* include/bits/stl_construct.h (destroy_at, construct_at): Guard
	with feature test macros instead of just __cplusplus.
---
 libstdc++-v3/include/bits/stl_construct.h | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/libstdc++-v3/include/bits/stl_construct.h b/libstdc++-v3/include/bits/stl_construct.h
index 7c394072b50..dc08fb7ea33 100644
--- a/libstdc++-v3/include/bits/stl_construct.h
+++ b/libstdc++-v3/include/bits/stl_construct.h
@@ -74,7 +74,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
-#if __cplusplus >= 201703L
+#if __glibcxx_raw_memory_algorithms // >= C++17
   template <typename _Tp>
     _GLIBCXX20_CONSTEXPR inline void
     destroy_at(_Tp* __location)
@@ -88,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	__location->~_Tp();
     }
 
-#if __cplusplus >= 202002L
+#if __cpp_constexpr_dynamic_alloc // >= C++20
   template<typename _Tp, typename... _Args>
     constexpr auto
     construct_at(_Tp* __location, _Args&&... __args)
@@ -108,7 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline void
     _Construct(_Tp* __p, _Args&&... __args)
     {
-#if __cplusplus >= 202002L
+#if __cpp_constexpr_dynamic_alloc // >= C++20
       if (std::__is_constant_evaluated())
 	{
 	  // Allow std::_Construct to be used in constant expressions.
@@ -145,7 +145,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _GLIBCXX14_CONSTEXPR inline void
     _Destroy(_Tp* __pointer)
     {
-#if __cplusplus > 201703L
+#if __cpp_constexpr_dynamic_alloc // >= C++20
       std::destroy_at(__pointer);
 #else
       __pointer->~_Tp();
@@ -188,7 +188,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static_assert(is_destructible<_Value_type>::value,
 		    "value type is destructible");
 #endif
-#if __cplusplus >= 202002L
+#if __cpp_constexpr_dynamic_alloc // >= C++20
       if (std::__is_constant_evaluated())
 	return std::_Destroy_aux<false>::__destroy(__first, __last);
 #endif
@@ -237,7 +237,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static_assert(is_destructible<_Value_type>::value,
 		    "value type is destructible");
 #endif
-#if __cplusplus >= 202002L
+#if __cpp_constexpr_dynamic_alloc // >= C++20
       if (std::__is_constant_evaluated())
 	return std::_Destroy_n_aux<false>::__destroy_n(__first, __count);
 #endif
@@ -245,7 +245,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	__destroy_n(__first, __count);
     }
 
-#if __cplusplus >= 201703L
+#if __glibcxx_raw_memory_algorithms // >= C++17
   template <typename _ForwardIterator>
     _GLIBCXX20_CONSTEXPR inline void
     destroy(_ForwardIterator __first, _ForwardIterator __last)
-- 
2.44.0