re PR libstdc++/77641 (std::variant copy-initialization fails for type with non-trivial constexpr ctor)
PR libstdc++/77641
* include/std/variant (_Variant_storage::_Variant_storage):
Change _Variant_storage's union to be default constructible.
* testsuite/20_util/variant/compile.cc: New test.
From-SVN: r240340
