From b46e4afaff5b2cfd23b6d4d364d12f3f7ec07f78 Mon Sep 17 00:00:00 2001
From: Martin Uecker <uecker@tugraz.at>
Date: Sun, 21 Dec 2025 19:10:56 +0100
Subject: [PATCH] c: Fix ICE for invalid code with variadic and old-school
 prototypes [PR121507]

When mixing old-school definition without prototype and new C23
variadic functions without named argument, there can be an ICE
when trying to form the composite type.  Avoid this by letting it
fail later due to incompatible types.

	PR c/121507

gcc/c/ChangeLog:
	* c-decl.cc (start_function): Adapt condition.

gcc/testsuite/ChangeLog:
	* gcc.dg/pr121507.c: New test.
---
 gcc/c/c-decl.cc                 |  3 ++-
 gcc/testsuite/gcc.dg/pr121507.c | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.dg/pr121507.c

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;
+}
+