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

[Ville Voutilainen]

On Mon, 5 Oct 2020 at 01:15, Ville Voutilainen
<ville.voutilainen@gmail.com> wrote:
> The patch is borked, doesn't pass tests, fixing...

Unborked, ok for trunk if full testsuite passes?

2020-10-05  Ville Voutilainen  <ville.voutilainen@gmail.com>

    PR libstdc++/95904
    * include/std/variant (__deduce_visit_result): Add a nested ::type.
    (__gen_vtable_impl</*base case*/>::_S_apply):
    Check the visitor return type.
    (__same_types): New.
    (__check_visitor_result): Likewise.
    (__check_visitor_results): Likewise.
    (visit(_Visitor&&, _Variants&&...)): Use __check_visitor_results
    in case we're visiting just one variant.
    * testsuite/20_util/variant/visit_neg.cc: Adjust.
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