libstdc++: Use __cpp_concepts instead of custom macro [PR103891]

With the new value of __cpp_concepts required by P2493, we can test
whether the compiler supports conditionally trivial special members.
This allows us to remove the workaround that disables fully-constexpr
std::variant for Clang. Now it should work for non-GCC compilers (such
as future releases of Clang) that support conditionally trivial
destructors and define the new value of __cpp_concepts.

libstdc++-v3/ChangeLog:

	PR libstdc++/103891
	* include/bits/c++config (_GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS):
	Remove.
	* include/std/variant: Check feature test macros instead.
	* include/std/version: Likewise.

1 files changed, 6 insertions, 8 deletions

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index c41f9f2..beed396 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -44,24 +44,22 @@
 #include <bits/stl_iterator_base_funcs.h>
 #include <bits/stl_construct.h>
 #include <bits/utility.h> // in_place_index_t
-#ifndef _GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS
-# include <ext/aligned_buffer.h>
-#endif
 #if __cplusplus >= 202002L
 # include <compare>
 #endif
 
-namespace std _GLIBCXX_VISIBILITY(default)
-{
-_GLIBCXX_BEGIN_NAMESPACE_VERSION
-
-#ifdef _GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS
+#if __cpp_concepts >= 202002L && __cpp_constexpr >= 201811L
 // P2231R1 constexpr needs constexpr unions and constrained destructors.
 # define __cpp_lib_variant 202106L
 #else
+# include <ext/aligned_buffer.h> // Use __aligned_membuf instead of union.
 # define __cpp_lib_variant 202102L
 #endif
 
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
   template<typename... _Types> class tuple;
   template<typename... _Types> class variant;
   template <typename> struct hash;