[PATCH] Implement std::visit<R> for C++2a (P0655R1)

[Ville Voutilainen]

On Sat, 6 Apr 2019 at 02:55, Ville Voutilainen
<ville.voutilainen@gmail.com> wrote:

> Just in case that cast looks scary: the implicit conversion is also
> deep down in __visit_invoke, so
> we do actually require implicit convertibility as we are supposed to.
> If that's still too scary,
> we can just do
>
> -      return (_Res)
> +      if constexpr (!is_void_v<_Res>)
> +        return
> +         __do_visit<false, false>(std::forward<_Visitor>(__visitor),
> +                                  std::forward<_Variants>(__variants)...);
> +      else
>
> like in the attached patch.

Okay, I merged that with the original. I also renamed the neg-test, so
here goes the hopefully final variant change for GCC 9:

2019-04-06  Ville Voutilainen  <ville.voutilainen@gmail.com>

    Fix visit<R> for variant.
    * include/std/variant (__do_visit): Add a template parameter
    for enforcing same return types for visit.
    (__gen_vtable_impl): Likewise.
    (_S_apply_single_alt): Adjust.
    (__visit_invoke_impl): New. Handle casting to void.
    (__do_visit_invoke): New. Enforces same return types.
    (__do_visit_invoke_r): New. Converts return types.
    (__visit_invoke): Adjust.
    (__gen_vtable):  Add a template parameter for enforcing
    same return types for visit.
    * testsuite/20_util/variant/visit_r.cc: Add a test for a visitor with
    different return types.
    * testsuite/20_util/variant/visit_neg.cc: New. Ensures that
    visitors with different return types don't accidentally
    compile with regular visitation.
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