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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/103891] clang-13 fails to compile libstdc++'s
 std::variant<std::vector<int>>: error: attempt to use a deleted function
Date: Tue, 04 Jan 2022 10:17:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103891

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I suppose we could just do:

--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -54,7 +54,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION

-#if __cplusplus >=3D 202002L && __cpp_concepts
+#if __cplusplus >=3D 202002L && __cpp_concepts && __GNUC__ >=3D 12
 // P2231R1 constexpr needs constexpr unions and constrained destructors.
 # define __cpp_lib_variant 202106L
 #else

And then improve it later if GCC updates __cpp_concepts=