[PATCH] Avoid copies in std::scoped_allocator_adaptor piecewise construction

[Jonathan Wakely <jwakely at redhat dot com>]

While looking at LWG  2511 I realised that we can prevent
scoped_allocator_adaptor::construct(pair<T, U>*, ...) from making any
copies internally. The transformed tuples that get passed to the
std::pair constructor (with additional allocator arguments) can be
tuples of references even if the incoming tuples are not.

See P0475R0 (which will be in the pre-Issaquah mailing) for a proposal
to resolve LWG 2511 by requiring this behaviour.

	* include/std/scoped_allocator (scoped_allocator_adaptor): Forward
	piecewise construction arguments as tuples of references, to avoid
	copies (related to LWG 2511).
	* testsuite/20_util/scoped_allocator/construct_pair.cc: New test.

Tested powerpc64le-linux, committed to trunk.

Attachment: patch.txt
Description: Text document