The rule change in the title matches GCC's current behavior, so no change
was needed.  But the paper also makes 'typename' optional in a
requirement-parameter-list, so this implements that.

gcc/cp/ChangeLog
2020-02-28  Jason Merrill  <jason@redhat.com>

	Implement P2092R0, Disambiguating Nested-Requirements
	* parser.c (cp_parser_requirement_parameter_list): Pass
	CP_PARSER_FLAGS_TYPENAME_OPTIONAL.