PR libstdc++/86963 Implement LWG 2729 constraints on tuple assignment

PR libstdc++/86963 Implement LWG 2729 constraints on tuple assignment

	PR libstdc++/86963
	* include/std/tuple (__tuple_base): New class template with deleted
	copy assignment operator.
	(tuple, tuple<_T1, _T2>): Derive from __tuple_base<tuple> so that
	implicit copy/move assignment operator will be deleted/suppressed.
	(tuple::__assignable, tuple<_T1, _T2>::__assignable): New helper
	functions for SFINAE constraints on assignment operators.
	(tuple::__nothrow_assignable, tuple<_T1, _T2>::__nothrow_assignable):
	New helper functions for exception specifications.
	(tuple::operator=(const tuple&), tuple::operator=(tuple&&))
	(tuple<_T1, _T2>::operator=(const tuple&))
	(tuple<_T1, _T2>::operator=(tuple&&)): Change parameter types to
	__nonesuch_no_braces when the operator should be defined implicitly.
	Use __nothrow_assignable for exception specifications.
	(tuple::operator=(const tuple<_UElements...>&))
	(tuple::operator=(tuple<_UElements...>&&))
	(tuple<_T1, _T2>::operator=(const tuple<_U1, _U2>&))
	(tuple<_T1, _T2>::operator=(tuple<_U1, _U2>&&))
	(tuple<_T1, _T2>::operator=(const pair<_U1, _U2>&))
	(tuple<_T1, _T2>::operator=(pair<_U1, _U2>&&)): Constrain using
	__assignable and use __nothrow_assignable for exception
	specifications.
	* python/libstdcxx/v6/printers.py (is_specialization_of): Accept
	gdb.Type as first argument, instead of a string.
	(StdTuplePrinter._iterator._is_nonempty_tuple): New method to check
	tuple for expected structure.
	(StdTuplePrinter._iterator.__init__): Use _is_nonempty_tuple.
	* testsuite/20_util/tuple/dr2729.cc: New test.
	* testsuite/20_util/tuple/element_access/get_neg.cc: Change dg-error
	to dg-prune-output.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@263625 138bc75d-0d04-0410-961f-82ee72b054a4