diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index 51a62c800f7..3b45cfda0ff 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -1692,7 +1692,7 @@ function_types_compatible_p (const_tree f1, const_tree f2,
 
   if (args1 == NULL_TREE)
     {
-      if (!self_promoting_args_p (args2))
+      if (flag_isoc2x ? stdarg_p (f2) : !self_promoting_args_p (args2))
 	return 0;
       /* If one of these types comes from a non-prototype fn definition,
 	 compare that with the other type's arglist.
@@ -1705,7 +1705,7 @@ function_types_compatible_p (const_tree f1, const_tree f2,
     }
   if (args2 == NULL_TREE)
     {
-      if (!self_promoting_args_p (args1))
+      if (flag_isoc2x ? stdarg_p (f1) : !self_promoting_args_p (args1))
 	return 0;
       if (TYPE_ACTUAL_ARG_TYPES (f2)
 	  && type_lists_compatible_p (args1, TYPE_ACTUAL_ARG_TYPES (f2),
diff --git a/gcc/testsuite/gcc.dg/c11-unproto-1.c b/gcc/testsuite/gcc.dg/c11-unproto-1.c
new file mode 100644
index 00000000000..ea9e807a68e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c11-unproto-1.c
@@ -0,0 +1,24 @@
+/* Test compatibility of unprototyped and prototyped function types (C2x makes
+   the case of types affected by default argument promotions compatible).  Test
+   valid-in-C2x usages are not accepted for C11.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+
+void f1 (); /* { dg-message "previous declaration" } */
+void f1 (float); /* { dg-error "conflicting types" } */
+/* { dg-message "default promotion" "" { target *-*-* } .-1 } */
+
+void f2 (float); /* { dg-message "previous declaration" } */
+void f2 (); /* { dg-error "conflicting types" } */
+/* { dg-message "default promotion" "" { target *-*-* } .-1 } */
+
+void f3 (); /* { dg-message "previous declaration" } */
+void f3 (char); /* { dg-error "conflicting types" } */
+/* { dg-message "default promotion" "" { target *-*-* } .-1 } */
+
+void f4 (char); /* { dg-message "previous declaration" } */
+void f4 (); /* { dg-error "conflicting types" } */
+/* { dg-message "default promotion" "" { target *-*-* } .-1 } */
+
+/* Built-in function case.  */
+float sqrtf (); /* { dg-warning "conflicting types for built-in function" } */
diff --git a/gcc/testsuite/gcc.dg/c11-unproto-2.c b/gcc/testsuite/gcc.dg/c11-unproto-2.c
new file mode 100644
index 00000000000..0557ae3f5cb
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c11-unproto-2.c
@@ -0,0 +1,21 @@
+/* Test compatibility of unprototyped and prototyped function types (C2x makes
+   the case of types affected by default argument promotions compatible).  Test
+   invalid-in-C2x usages, in C11 mode.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+
+void f1 (); /* { dg-message "previous declaration" } */
+void f1 (int, ...); /* { dg-error "conflicting types" } */
+/* { dg-message "ellipsis" "" { target *-*-* } .-1 } */
+
+void f2 (int, ...); /* { dg-message "previous declaration" } */
+void f2 (); /* { dg-error "conflicting types" } */
+/* { dg-message "ellipsis" "" { target *-*-* } .-1 } */
+
+void f3 (); /* { dg-message "previous declaration" } */
+void f3 (char, ...); /* { dg-error "conflicting types" } */
+/* { dg-message "ellipsis" "" { target *-*-* } .-1 } */
+
+void f4 (char, ...); /* { dg-message "previous declaration" } */
+void f4 (); /* { dg-error "conflicting types" } */
+/* { dg-message "ellipsis" "" { target *-*-* } .-1 } */
diff --git a/gcc/testsuite/gcc.dg/c2x-unproto-1.c b/gcc/testsuite/gcc.dg/c2x-unproto-1.c
new file mode 100644
index 00000000000..45d68f2c292
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2x-unproto-1.c
@@ -0,0 +1,20 @@
+/* Test compatibility of unprototyped and prototyped function types (C2x makes
+   the case of types affected by default argument promotions compatible).  Test
+   valid-in-C2x usages.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+void f1 ();
+void f1 (float);
+
+void f2 (float);
+void f2 ();
+
+void f3 ();
+void f3 (char);
+
+void f4 (char);
+void f4 ();
+
+/* Built-in function case.  */
+float sqrtf ();
diff --git a/gcc/testsuite/gcc.dg/c2x-unproto-2.c b/gcc/testsuite/gcc.dg/c2x-unproto-2.c
new file mode 100644
index 00000000000..f826b7c3ac8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2x-unproto-2.c
@@ -0,0 +1,21 @@
+/* Test compatibility of unprototyped and prototyped function types (C2x makes
+   the case of types affected by default argument promotions compatible).  Test
+   invalid-in-C2x usages.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+void f1 (); /* { dg-message "previous declaration" } */
+void f1 (int, ...); /* { dg-error "conflicting types" } */
+/* { dg-message "ellipsis" "" { target *-*-* } .-1 } */
+
+void f2 (int, ...); /* { dg-message "previous declaration" } */
+void f2 (); /* { dg-error "conflicting types" } */
+/* { dg-message "ellipsis" "" { target *-*-* } .-1 } */
+
+void f3 (); /* { dg-message "previous declaration" } */
+void f3 (char, ...); /* { dg-error "conflicting types" } */
+/* { dg-message "ellipsis" "" { target *-*-* } .-1 } */
+
+void f4 (char, ...); /* { dg-message "previous declaration" } */
+void f4 (); /* { dg-error "conflicting types" } */
+/* { dg-message "ellipsis" "" { target *-*-* } .-1 } */