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

[Jonathan Wakely]

Tested x86_64-linux (and clang-12.0.1), pushed to trunk.

-- >8 --

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.
---
 libstdc++-v3/include/bits/c++config |  5 -----
 libstdc++-v3/include/std/variant    | 18 ++++++++----------
 libstdc++-v3/include/std/version    |  4 ++--
 3 files changed, 10 insertions(+), 17 deletions(-)

diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index b197349f976..c64b61b3c90 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -810,11 +810,6 @@ namespace std
 
 #undef _GLIBCXX_HAS_BUILTIN
 
-#if __cplusplus >= 202002L && __cpp_concepts && __GNUC__ >= 12
-// XXX workaround for missing feature test macro for P0848R3 (see P2493R0).
-# define _GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS 1
-#endif
-
 // PSTL configuration
 
 #if __cplusplus >= 201703L
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index c41f9f27e00..beed396fccb 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
 
+#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
 
-#ifdef _GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS
-// P2231R1 constexpr needs constexpr unions and constrained destructors.
-# define __cpp_lib_variant 202106L
-#else
-# define __cpp_lib_variant 202102L
-#endif
-
   template<typename... _Types> class tuple;
   template<typename... _Types> class variant;
   template <typename> struct hash;
diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index 30e04b55555..24311ee05c8 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -175,7 +175,7 @@
 # define __cpp_lib_to_chars 201611L
 #endif
 #define __cpp_lib_unordered_map_try_emplace 201411L
-#ifndef _GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS
+#if !(__cplusplus >= 202002L && __cpp_concepts >= 202002L)
 // N.B. updated value in C++20
 # define __cpp_lib_variant 202102L
 #endif
@@ -292,7 +292,7 @@
 # endif
 #define __cpp_lib_to_address 201711L
 #define __cpp_lib_to_array 201907L
-#ifdef _GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS
+#if __cplusplus >= 202002L && __cpp_concepts >= 202002L
 # define __cpp_lib_variant 202106L
 #endif
 #endif
-- 
2.34.1