c++: generic lambda in default template argument [PR88313]

c++: generic lambda in default template argument [PR88313]

Here we're rejecting the generic lambda inside the default template
argument ultimately because auto_is_implicit_function_template_parm_p
doesn't get set during parsing of the lambda's parameter list, due
to the !processing_template_parmlist restriction.  But when parsing a
lambda parameter list we should always set that flag regardless of where
the lambda appears.  This patch makes sure of this via a local lambda_p
flag.

	PR c++/88313

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_lambda_declarator_opt): Pass
	lambda_p=true to cp_parser_parameter_declaration_clause.
	(cp_parser_direct_declarator): Pass lambda_p=false to
	to cp_parser_parameter_declaration_clause.
	(cp_parser_parameter_declaration_clause): Add bool lambda_p
	parameter.  Consider lambda_p instead of current_class_type
	when setting parser->auto_is_implicit_function_template_parm_p.
	Don't consider processing_template_parmlist.
	(cp_parser_requirement_parameter_list): Pass lambda_p=false
	to cp_parser_parameter_declaration_clause.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-targ6.C: New test.

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