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

	PR c++/92236
	Defer evaluation of concept checks so that static assertions can
	emit more detailed diagnostics.

gcc/cp/
	* constexpr.c (cxx_eval_call_expression): Handle concept checks.
	(cxx_eval_constant_expression): Diagnose misuse of function concepts
	as template-id expressions. Follow the usual return path for results.
	(cxx_eval_outermost_constant_expr): Avoid calling
	cp_get_callee_fndecl_nofold for function concepts.
	* constraint.cc (build_function_check): Fully type the concept check
	so that we don't ICE in conversions.
	* cp-gimplify.c (cp_genericize_r) [CALL_EXPR]: Handle concept checks.
	[TEMPLATE_ID_EXPR] Likewise.
	* cvt.c (convert_to_void): Always evaluate concept checks so we don't
	accidentally ignore them. Substitution during satisfaction can make
	a program ill-formed (example in g++.dg/cpp2a/concepts6.C).
	* pt.c (tsubst_copy_and_build): [CALL_EXPR]: Don't evaluate concepts.
	[TEMPLATE_ID_EXPR]: Likewise.
	* semantics.c (finish_call_expr): Don't evaluate concepts.
	(finish_id_expression_1): Likewise.
	(finish_static_assert): Preserve the original condition so we can
	diagnose concept errors when a check returns false.

gcc/testsuite/
	* g++.dg/cpp2a/concepts-iconv1.C: Update diagnostics.
	* g++.dg/cpp2a/concepts-requires5.C: Likewise.
	* g++.dg/cpp2a/concepts6.C: New test.

From-SVN: r278775