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

[Ville Voutilainen]

On Sat, 6 Apr 2019 at 02:19, Ville Voutilainen
<ville.voutilainen@gmail.com> wrote:

> And this patch fixes the existing visitation so that we don't
> over-eagerly cast to void. The main gist of it is
>
> -        else if constexpr (is_same_v<_Result_type, void>)
> +        else if constexpr (!__same_return_types &&
> +                          is_same_v<_Result_type, void>)
>
> and the new ensure-test is adjusted to use void as the return type of
> the first visitor.

..and this patch makes cv void work as the return type. The important
bits are (incrementally)

-               is_same_v<_Result_type, void>)
+               is_void_v<_Result_type>)

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.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: variant-visit_r_3.diff
Type: text/x-patch
Size: 8772 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20190405/84e00967/attachment.bin>