re PR libstdc++/66338 (std::forward_as_tuple() issue with single argument)

2016-05-24  Ville Voutilainen  <ville.voutilainen@gmail.com>

	PR libstdc++/66338
	* include/std/tuple (_TMC): Add a check for _NotSameTuple.
	* include/std/tuple (tuple(_UElements&&...)): Remove the separate
	check for _NotSameTuple.
	* include/std/tuple (_TMCT): New.
	* include/std/tuple (tuple(const tuple<_UElements...>&)): Use it.
	* include/std/tuple (tuple(tuple<_UElements...>&&)): Likewise.
	* include/std/tuple (tuple(allocator_arg_t, const _Alloc&,
	      const tuple<_UElements...>&)): Likewise.
	* include/std/tuple (tuple(allocator_arg_t, const _Alloc&,
	      tuple<_UElements...>&&)): Likewise.
	* testsuite/20_util/tuple/cons/66338.cc: New.

From-SVN: r236822