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

[Ville Voutilainen]

On Sat, 6 Apr 2019 at 02:48, Ville Voutilainen
<ville.voutilainen@gmail.com> wrote:
> and
>
> -      return __do_visit<false, false>(std::forward<_Visitor>(__visitor),
> -                      std::forward<_Variants>(__variants)...);
> +      return (_Res)
> +    __do_visit<false, false>(std::forward<_Visitor>(__visitor),
> +                 std::forward<_Variants>(__variants)...);
>
> and the visit_r test is amended to also test visitation of a visitor
> with a const void result, when the visitor
> itself does not return void.

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.
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