[PATCH] Implement std::launder for C++17

[Jonathan Wakely <jwakely at redhat dot com>]

This implements std::launder, using Jakub's new __builtin_launder.

In order to allow our headers to be used with Clang (which doesn't
implement all our new built-ins yet) I've added a couple of checks
using Clang's __has_builtin.

My initial version of std::launder used
static_assert(!is_funcion_v<_Tp> && !is_void_v<_Tp>) which required
including <type_traits>, and that uses the new built-in
__has_unique_object_representations, so I guarded that too. In the end
I decided not to include <type_traits> in <new> and dealt with the
function and void cases using deleted overloads of std::launder
instead.

	* doc/xml/manual/status_cxx2017.xml: Update status.
	* doc/html/*: Regenerate.
	* include/std/type_traits (has_unique_object_representations): Guard
	with __has_builtin check.
	* libsupc++/new (launder): Define for C++17.
	* testsuite/18_support/launder/1.cc: New test.
	* testsuite/18_support/launder/requirements.cc: New test.
	* testsuite/18_support/launder/requirements_neg.cc: New test.

Tested powerpc64le-linux, committed to trunk.

Attachment: patch.txt
Description: Text document