[PATCH] libstdc++: Diagnose visitors with different return types [PR95904]

[Ville Voutilainen]

On Sat, 17 Oct 2020 at 20:30, Stephan Bergmann <sbergman@redhat.com> wrote:

> Clang (with -std=c++17/20) now complains about
>
> > include/c++/11.0.0/variant:1032:10: error: no matching constructor for initialization of 'std::__nonesuch'
> >                 return __nonesuch{};
> >                        ^         ~~
> > 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;
> >     ^
>
> upon #include <variant>.  (And I think legitimately so, as __nonsuch is
> not dependent?)

This is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97449, I'll
commit a fix today.