[PATCH] libstdc++/77641 fix std::variant for literal types

[Jonathan Wakely <jwakely at redhat dot com>]

As noted in bugzilla PR 77641 I don't think is_literal_type is the
right condition for _Uninitialized, because a type can have a
non-trivial default constructor but still be literal, but that means
it can't be used in the union in _Variant_storage.

I'm not sure if the right condition is is_literal &&
is_trivially_default_constructible, or if this is enough:

	PR libstdc++/77641
	* include/std/variant (__detail::__variant::_Uninitialized): Check
	is_trivially_default_constructible_v instead of is_literal_type_v.
	* testsuite/20_util/variant/compile.cc: Test literal type with
	non-trivial default constructor.

Tim, are there case that this doesn't handle, where we need is_literal
as well? (bear in mind that is_trivially_default_constructible also
depends on trivially destructible).

Tested powerpc64le-linux with no failures.

Attachment: patch.txt
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