PR c++/91923 - failure-to-SFINAE with class type NTTP in C++17.

	* pt.c (invalid_nontype_parm_type_p): Only emit errors when
	tf_error.

	* g++.dg/cpp0x/nontype5.C: New test.

From-SVN: r276248

gcc/cp/ChangeLog

+2019-09-28  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/91923 - failure-to-SFINAE with class type NTTP in C++17.
+	* pt.c (invalid_nontype_parm_type_p): Only emit errors when
+	tf_error.
+
 2019-09-27  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/88203

gcc/cp/pt.c

@@ -25232,8 +25232,9 @@ invalid_nontype_parm_type_p (tree type, tsubst_flags_t complain)
     {
       if (cxx_dialect < cxx2a)
 	{
-	  error ("non-type template parameters of class type only available "
-		 "with %<-std=c++2a%> or %<-std=gnu++2a%>");
+	  if (complain & tf_error)
+	    error ("non-type template parameters of class type only available "
+		   "with %<-std=c++2a%> or %<-std=gnu++2a%>");
 	  return true;
 	}
       if (dependent_type_p (type))

gcc/testsuite/ChangeLog

+2019-09-28  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/91923 - failure-to-SFINAE with class type NTTP in C++17.
+	* g++.dg/cpp0x/nontype5.C: New test.
+
 2019-09-28  Alan Modra  <amodra@gmail.com>
 
 	PR testsuite/91676

gcc/testsuite/g++.dg/cpp0x/nontype5.C

+// PR c++/91923 - failure-to-SFINAE with class type NTTP in C++17.
+// { dg-do compile { target c++11 } }
+
+template<typename T>
+constexpr bool is_integral_(...) {
+    return false;
+}
+template<typename T, T = 1>
+constexpr bool is_integral_(long) {
+    return true;
+}
+
+static_assert(is_integral_<int>(42), "");
+static_assert(!is_integral_<void>(42), "");
+
+struct S {};
+static_assert(!is_integral_<S>(42), "");