pt.c (convert_template_argument): Add a diagnostic for the [temp.arg]/2 ambiguity case.

	* pt.c (convert_template_argument): Add a diagnostic for the
	[temp.arg]/2 ambiguity case.

	* g++.dg/cpp2a/nontype-class17.C: New test.

From-SVN: r271431

gcc/cp/ChangeLog

+2019-05-20  Marek Polacek  <polacek@redhat.com>
+
+	* pt.c (convert_template_argument): Add a diagnostic for the
+	[temp.arg]/2 ambiguity case.
+
 2019-05-20  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	* cp-tree.h: Remove remnants of CONV_NONCONVERTING.

gcc/cp/pt.c

@@ -7963,10 +7963,22 @@ convert_template_argument (tree parm,
 		     "parameter list for %qD",
 		     i + 1, in_decl);
 	      if (is_type)
-		inform (input_location,
-			"  expected a constant of type %qT, got %qT",
-			TREE_TYPE (parm),
-			(DECL_P (arg) ? DECL_NAME (arg) : orig_arg));
+		{
+		  /* The template argument is a type, but we're expecting
+		     an expression.  */
+		  inform (input_location,
+			  "  expected a constant of type %qT, got %qT",
+			  TREE_TYPE (parm),
+			  (DECL_P (arg) ? DECL_NAME (arg) : orig_arg));
+		  /* [temp.arg]/2: "In a template-argument, an ambiguity
+		     between a type-id and an expression is resolved to a
+		     type-id, regardless of the form of the corresponding
+		     template-parameter."  So give the user a clue.  */
+		  if (TREE_CODE (arg) == FUNCTION_TYPE)
+		    inform (input_location, "  ambiguous template argument "
+			    "for non-type template parameter is treated as "
+			    "function type");
+		}
 	      else if (requires_tmpl_type)
 		inform (input_location,
 			"  expected a class template, got %qE", orig_arg);

gcc/testsuite/ChangeLog

@@ -2,6 +2,8 @@
 
 	* g++.dg/ext/utf8-2.C: Accept both "char" and "char8_t" in aka.
 
+	* g++.dg/cpp2a/nontype-class17.C: New test.
+
 2019-05-20  Jeff Law  <law@redhat.com>
 
 	* gcc.dg/Wtype-limits-Wextra.c: Adjust expected output after

gcc/testsuite/g++.dg/cpp2a/nontype-class17.C

+// { dg-do compile { target c++2a } }
+
+template<auto>
+struct S { };
+
+struct R { };
+
+void
+g (void)
+{
+  S<R()> s; // { dg-error "mismatch" }
+// { dg-message "treated as function" "note" { target *-*-* } .-1 }
+  S<R{}> s2;
+  S<int()> s3; // { dg-error "mismatch" }
+// { dg-message "treated as function" "note" { target *-*-* } .-1 }
+  S<int{}> s4;
+}