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

[Jonathan Wakely]

On 05/04/19 19:06 +0100, Jonathan Wakely wrote:
>	* doc/xml/manual/status_cxx2020.xml: Update status.
>	* include/std/variant (visit<R>): Define for C++2a (P0655R1).
>	* testsuite/20_util/variant/visit_r.cc: New test.

This implementation is wrong, the conversions to R need to happen for
each possible invocation, which makes it more complicated (we need to
duplicate some code). I'll fix it next week.