[Bug libstdc++/97449] New: libstdc++ <variant> cannot be compiled with clang after 3427e31331677ca826c5588c87924214f7e5c54b

["foom at fuhm dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97449

            Bug ID: 97449
           Summary: libstdc++ <variant> cannot be compiled with clang
                    after 3427e31331677ca826c5588c87924214f7e5c54b
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: foom at fuhm dot net
  Target Milestone: ---

After 3427e31331677ca826c5588c87924214f7e5c54b, "clang -std=c++17" (using
libstdc++ headers), on a file that has simply "#include <variant>" fails with
this error:

In file included from <source>:1:

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.0/../../../../include/c++/11.0.0/variant:1032:10:
error: no matching constructor for initialization of 'std::__nonesuch'
                return __nonesuch{};
                       ^         ~~

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.0/../../../../include/c++/11.0.0/type_traits:2953:5:
note: candidate constructor not viable: requires 1 argument, but 0 were
provided
    __nonesuch(__nonesuch const&) = delete;
    ^

(https://godbolt.org/z/951jYr)

I believe Clang is correct to reject this code, due to
http://eel.is/c++draft/temp.res#general-8.1
"""
The program is ill-formed, no diagnostic required, if:
- no valid specialization can be generated for a template or a substatement of
a constexpr if statement within a template and the template is not instantiated
"""

The invalid return statement probably needs to be made dependent on
__visit_ret_type_mismatch just like the static_assert.