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

gcc/cp/ChangeLog

 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.
+	* 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.
+
+2019-11-27  Andrew Sutton  <asutton@lock3software.com>
+
 	PR c++/92439
 	Improve quality of diagnostics for subexpressions that need parens.
 	* parser.c (cp_parser_requires_clause_opt): Add a flag to indicate

gcc/cp/constexpr.c

@@ -1672,6 +1672,10 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 			  bool lval,
 			  bool *non_constant_p, bool *overflow_p)
 {
+  /* Handle concept checks separately.  */
+  if (concept_check_p (t))
+    return evaluate_concept_check (t, tf_warning_or_error);
+
   location_t loc = cp_expr_loc_or_input_loc (t);
   tree fun = get_function_named_in_call (t);
   constexpr_call new_call
@@ -5645,14 +5649,26 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
       {
         /* We can evaluate template-id that refers to a concept only if
 	   the template arguments are non-dependent.  */
-	if (!concept_definition_p (TREE_OPERAND (t, 0)))
+	tree id = unpack_concept_check (t);
+	tree tmpl = TREE_OPERAND (id, 0);
+	if (!concept_definition_p (tmpl))
 	  internal_error ("unexpected template-id %qE", t);
 
+	if (function_concept_p (tmpl))
+	  {
+	    if (!ctx->quiet)
+	      error_at (cp_expr_loc_or_input_loc (t),
+			"function concept must be called");
+	    r = error_mark_node;
+	    break;
+	  }
+
 	if (!processing_template_decl)
-	  return evaluate_concept_check (t, tf_warning_or_error);
+	  r = evaluate_concept_check (t, tf_warning_or_error);
 	else
 	  *non_constant_p = true;
-	return t;
+
+	break;
       }
 
     case ASM_EXPR:
@@ -5809,12 +5825,16 @@ cxx_eval_outermost_constant_expr (tree t, bool allow_non_constant,
 	       || TREE_CODE (t) == AGGR_INIT_EXPR
 	       || TREE_CODE (t) == TARGET_EXPR))
     {
-      tree x = t;
-      if (TREE_CODE (x) == TARGET_EXPR)
-	x = TARGET_EXPR_INITIAL (x);
-      tree fndecl = cp_get_callee_fndecl_nofold (x);
-      if (fndecl && DECL_IMMEDIATE_FUNCTION_P (fndecl))
-	is_consteval = true;
+      /* For non-concept checks, determine if it is consteval.  */
+      if (!concept_check_p (t))
+	{
+	  tree x = t;
+	  if (TREE_CODE (x) == TARGET_EXPR)
+	    x = TARGET_EXPR_INITIAL (x);
+	  tree fndecl = cp_get_callee_fndecl_nofold (x);
+	  if (fndecl && DECL_IMMEDIATE_FUNCTION_P (fndecl))
+	    is_consteval = true;
+	}
     }
   if (AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type))
     {

gcc/cp/constraint.cc

@@ -1263,6 +1263,7 @@ build_function_check (tree tmpl, tree args, tsubst_flags_t /*complain*/)
   ++processing_template_decl;
   vec<tree, va_gc> *fargs = make_tree_vector ();
   tree call = build_min_nt_call_vec (id, fargs);
+  TREE_TYPE (call) = boolean_type_node;
   release_tree_vector (fargs);
   --processing_template_decl;
 
@@ -1397,6 +1398,7 @@ build_constrained_parameter (tree cnc, tree proto, tree args)
 
    Note that the constraints are neither reduced nor decomposed. That is
    done only after the requires clause has been parsed (or not).  */
+
 tree
 finish_shorthand_constraint (tree decl, tree constr)
 {

gcc/cp/cp-gimplify.c

@@ -1622,6 +1622,15 @@ cp_genericize_r (tree *stmt_p, int *walk_subtrees, void *data)
       break;
 
     case CALL_EXPR:
+      /* Evaluate function concept checks instead of treating them as
+	 normal functions.  */
+      if (concept_check_p (stmt))
+	{
+	  *stmt_p = evaluate_concept_check (stmt, tf_warning_or_error);
+	  * walk_subtrees = 0;
+	  break;
+	}
+
       if (!wtd->no_sanitize_p
 	  && sanitize_flags_p ((SANITIZE_NULL
 				| SANITIZE_ALIGNMENT | SANITIZE_VPTR)))
@@ -1679,6 +1688,13 @@ cp_genericize_r (tree *stmt_p, int *walk_subtrees, void *data)
 	TARGET_EXPR_NO_ELIDE (stmt) = 1;
       break;
 
+    case TEMPLATE_ID_EXPR:
+      gcc_assert (concept_check_p (stmt));
+      /* Emit the value of the concept check.  */
+      *stmt_p = evaluate_concept_check (stmt, tf_warning_or_error);
+      walk_subtrees = 0;
+      break;
+
     default:
       if (IS_TYPE_OR_DECL_P (stmt))
 	*walk_subtrees = 0;

gcc/cp/cvt.c

@@ -1135,6 +1135,12 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
         error_at (loc, "pseudo-destructor is not called");
       return error_mark_node;
     }
+
+  /* Explicitly evaluate void-converted concept checks since their
+     satisfaction may produce ill-formed programs.  */
+   if (concept_check_p (expr))
+     expr = evaluate_concept_check (expr, tf_warning_or_error);
+
   if (VOID_TYPE_P (TREE_TYPE (expr)))
     return expr;
   switch (TREE_CODE (expr))

gcc/cp/pt.c

@@ -18860,12 +18860,6 @@ tsubst_copy_and_build (tree t,
 	    if (function_concept_p (TREE_OPERAND (id, 0)))
 	      RETURN (id);
 
-	    /* Evaluate the concept, if needed.  */
-	    tree args = TREE_OPERAND (id, 1);
-	    if (!uses_template_parms (args)
-		&& !processing_constraint_expression_p ())
-	      RETURN (evaluate_concept_check (check, complain));
-
 	    RETURN (check);
 	  }
 
@@ -19650,11 +19644,6 @@ tsubst_copy_and_build (tree t,
 
 	    /* Ensure the result is wrapped as a call expression.  */
 	    ret = build_concept_check (tmpl, args, tf_warning_or_error);
-
-	    /* Possibly evaluate the check if it is non-dependent.   */
-	    if (!uses_template_parms (args)
-		&& !processing_constraint_expression_p ())
-	      ret = evaluate_concept_check (ret, complain);
 	  }
 	else
 	  ret = finish_call_expr (function, &call_args,

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)
 	{

gcc/testsuite/ChangeLog

 2019-11-27  Andrew Sutton  <asutton@lock3software.com>
 
+	PR c++/92236
+	* g++.dg/cpp2a/concepts-iconv1.C: Update diagnostics.
+	* g++.dg/cpp2a/concepts-requires5.C: Likewise.
+	* g++.dg/cpp2a/concepts6.C: New test.
+
+2019-11-27  Andrew Sutton  <asutton@lock3software.com>
+
 	PR c++/92439
 	* g++.dg/cpp2a/concepts-requires20.C: New.
 

gcc/testsuite/g++.dg/cpp2a/concepts-iconv1.C

@@ -7,16 +7,16 @@ int foo(int x)
 {
     return x;
 }
- 
+
 template <typename T>
 concept C1 = requires (T x) {
-    {foo(x)} -> Same<int&>;
+    {foo(x)} -> Same<int&>; // { dg-error "placeholder constraints" }
 };
 
 template <typename T>
 concept C2 = requires (T x) {
-    {foo(x)} -> Same<void>;
+    {foo(x)} -> Same<void>; // { dg-error "placeholder constraints" }
 };
- 
+
 static_assert( C1<int> );	// { dg-error "assert" }
 static_assert( C2<int> );	// { dg-error "assert" }

gcc/testsuite/g++.dg/cpp2a/concepts-requires5.C

@@ -33,13 +33,13 @@ int driver_1()
 // Test implicit conversion requirements
 
 template<typename T, typename U>
-concept ConvertibleTo = requires(T& t) { {t} -> U&; };
+concept ConvertibleTo = requires(T& t) { {t} -> U&; }; // { dg-error "inaccessible" }
 
 struct B { };
 class D : /*private*/ B { };
 
 void driver_2()
 {
-  static_assert(ConvertibleTo<D, B>()); // { dg-error "cannot be used as a function" }
+  static_assert(ConvertibleTo<D, B>()); // { dg-error "cannot call" }
   static_assert(ConvertibleTo<D, B>); // { dg-error "static assertion failed" }
 }