c++: Fix ICE on mangling invalid compound requirement [PR120618]

This testcase caused an ICE when mangling the invalid type-constraint in
write_requirement since write_type_constraint expects a TEMPLATE_TYPE_PARM.

Setting the trailing return type to NULL_TREE when a
return-type-requirement is found in place of a type-constraint prevents the
failed assertion in write_requirement. It also allows the invalid
constraint to be satisfied in some contexts to prevent redundant errors,
e.g. in concepts-requires5.C.

Bootstrapped and tested on x86_64-linux-gnu.

	PR c++/120618

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_compound_requirement): Set type to
	NULL_TREE for invalid type-constraint.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires5.C: Don't require
	redundant diagnostic in static assertion.
	* g++.dg/concepts/pr120618.C: New test.

Suggested-by: Jason Merrill <jason@redhat.com>

0 files changed, 0 insertions, 0 deletions