[PATCH] Make std::invoke usable in constant expressions

[Jonathan Wakely <jwakely at redhat dot com>]

	* include/std/functional (invoke): Add constexpr for C++20.
	* include/std/version (__cpp_lib_constexpr_invoke): Define.
	* testsuite/20_util/function_objects/invoke/constexpr.cc: New test.

This is an easy one, because I already made std::__invoke constexpr,
so all that's needed for C++20 is to add _GLIBCXX20_CONSTEXPR to the
public std::invoke function that calls std::__invoke.

Tested powerpc64le-linux, committed to trunk.

Attachment: patch.txt
Description: Text document