diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index fb17db4cb8e..89ccf820570 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -10787,7 +10787,8 @@ start_function (struct c_declspecs *declspecs, struct c_declarator *declarator,
       tree newrt = TREE_TYPE (newtype);
       if (old_decl != NULL_TREE
 	  && TREE_CODE (oldtype) == FUNCTION_TYPE
-	  && comptypes (oldrt, newrt))
+	  && comptypes (oldrt, newrt)
+	  && !TYPE_NO_NAMED_ARGS_STDARG_P (oldtype))
 	{
 	  if (stdarg_p (oldtype))
 	    {
diff --git a/gcc/testsuite/gcc.dg/pr121507.c b/gcc/testsuite/gcc.dg/pr121507.c
new file mode 100644
index 00000000000..2b9516aaffe
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr121507.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-std=c23" } */
+
+int * xmalloc(...);	
+int * xmalloc(n)	/* { dg-error "conflicting types" } */
+        int n;		/* { dg-warning "old-style function definition" "" { target *-*-* } .-1 } */
+{
+  return 0;
+}
+