nonesuch is insufficiently useless (lwg2996)

nonesuch is insufficiently useless (lwg2996)

2019-05-14  Nina Dinka Ranns  <dinka.ranns@gmail.com>

	nonesuch is insufficiently useless (lwg2996)
	* include/std/type_traits (struct __nonesuch): Added private base
	class to make __nonesuch not an aggregate and removed deleted default
	constructor.
	* include/bits/stl_pair.h (struct __nonesuch_no_braces): Removed.
	(operator=(const pair&)): Use __nonesuch instead of
	__nonesuch_no_braces.
	(operator=(pair&&)): Likewise
	* include/std/tuple (operator=(const tuple&)): Use __nonesuch instead
	of __nonesuch_no_braces.
	(operator=(tuple&&)): Likewise
	* include/experimental/type_traits (struct nonesuch): Added private
	base class to make nonesuch not an aggregate and removed deleted
	default constructor.
	* testsuite/20_util/nonesuch/nonesuch.cc: New.
	* testsuite/experimental/type_traits/nonesuch.cc: New.

From-SVN: r271175