[PATCH] Define macro to simplify std::_Not_fn definition

[Jonathan Wakely <jwakely at redhat dot com>]

While updating our C++17 features to add the new std::invoke_result
trait I got annoyed with the repetition in std::_Not_fn, as it makes
it hard to check it's defined correctly. Specifically, we need to
repeat the cv-qualifier and ref-qualifiers in multiple places in each
overload.

This introduces a macro that is expanded with one of "&" or "const &"
or "&&" or "const &&" and so ensures it's used consistently.

This only affects C++17 std::not_fn and std::experimental::not_fn so
I'm committing it now.

	* include/std/functional (_Not_fn): Define macro to simplify
	repetitive function definitions.

Tested powerpc64le-linux, committed to trunk.

Attachment: patch.txt
Description: Text document