2019-11-27  Andrew Sutton  <asutton@lock3software.com>

	PR c++/92439
	Improve quality of diagnostics for subexpressions that need parens.

gcc/cp/
	* parser.c (cp_parser_requires_clause_opt): Add a flag to indicate
	when parsing a requires-clause before lambda parameters, and...
	(cp_parser_lambda_declarator_opt): ... use that here ...
	(cp_parser_type_parameter): ... and here ...
	(cp_parser_late_return_type_opt): ... and here ...
	(cp_parser_explicit_template_declaration): ... and here.
	(cp_parser_diagnose_ungrouped_constraint_plain): Adjust the message
	because this can apply to subexpressions that are not immediately
	after a requires-clause.
	(cp_parser_diagnose_ungrouped_constraint_rich): Likewise.
	(primary_constraint_error): New.
	(cp_parser_constraint_requires_parens): New.
	(cp_parser_unary_constraint_requires_parens): New.
	(cp_parser_constraint_primary_expression): Check for unary expressions
	before parsing the primary expression. Also check for binary and
	postfix operators after a successful parse of the primary expression.
	Force a re-parse if the result would form a lower-precedence string.
	(cp_parser_constraint_logical_and_expression): Propagate lambda flag;
	move checks for ill-formed constraints into the constraint primary
	expression.
	(cp_parser_constraint_logical_or_expression): Likewise.
	(cp_parser_requires_clause_expression): Propagate lambda flag.

gcc/testsuite/
	* g++.dg/cpp2a/concepts-requires20.C: New.

From-SVN: r278774