re PR c++/92236 ([concepts] Explain non-satisfaction in static_assert)

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

1 files changed, 9 insertions, 11 deletions

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 6c4785c..4a5479c 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -2605,10 +2605,6 @@ finish_call_expr (tree fn, vec<tree, va_gc> **args, bool disallow_virtual,
 
       /* Ensure the result is wrapped as a call expression.  */
       result = build_concept_check (tmpl, args, tf_warning_or_error);
-
-      /* Evaluate the check if it is non-dependent.   */
-      if (!uses_template_parms (args))
-	result = evaluate_concept_check (result, complain);
     }
   else if (is_overloaded_fn (fn))
     {
@@ -3890,13 +3886,8 @@ finish_id_expression_1 (tree id_expression,
 	}
       else if (concept_check_p (decl))
 	{
-	  /* If this is a standard or variable concept check, potentially
-	     evaluate it. Function concepts need to be called as functions,
-	     so don't try evaluating them here.  */
-	  tree tmpl = TREE_OPERAND (decl, 0);
-	  tree args = TREE_OPERAND (decl, 1);
-	  if (!function_concept_p (tmpl) && !uses_template_parms (args))
-	    decl = evaluate_concept_check (decl, tf_warning_or_error);
+	  /* Nothing more to do. All of the analysis for concept checks
+	     is done by build_conept_id, called from the parser.  */
 	}
       else if (scope)
 	{
@@ -9564,6 +9555,9 @@ finish_static_assert (tree condition, tree message, location_t location,
       return;
     }
 
+  /* Save the condition in case it was a concept check.  */
+  tree orig_condition = condition;
+
   /* Fold the expression and convert it to a boolean value. */
   condition = perform_implicit_conversion_flags (boolean_type_node, condition,
 						 complain, LOOKUP_NORMAL);
@@ -9590,6 +9584,10 @@ finish_static_assert (tree condition, tree message, location_t location,
 	  else
             error ("static assertion failed: %s",
 		   TREE_STRING_POINTER (message));
+
+	  /* Actually explain the failure if this is a concept check.  */
+	  if (concept_check_p (orig_condition))
+	    diagnose_constraints (location, orig_condition, NULL_TREE);
 	}
       else if (condition && condition != error_mark_node)
 	{