--- gcc/gimple-range-op.cc.jj	2026-01-02 09:56:10.182336262 +0100
+++ gcc/gimple-range-op.cc	2026-01-09 15:00:01.166808500 +0100
@@ -1410,6 +1410,8 @@ gimple_range_op_handler::maybe_builtin_c
   switch (func)
     {
     case CFN_BUILT_IN_CONSTANT_P:
+      if (gimple_call_num_args (call) != 1)
+	return;
       m_op1 = gimple_call_arg (call, 0);
       if (irange::supports_p (TREE_TYPE (m_op1)))
 	m_operator = &op_cfn_constant_p;
@@ -1420,21 +1422,29 @@ gimple_range_op_handler::maybe_builtin_c
       break;
 
     CASE_FLT_FN (CFN_BUILT_IN_SIGNBIT):
+      if (gimple_call_num_args (call) != 1)
+	return;
       m_op1 = gimple_call_arg (call, 0);
       m_operator = &op_cfn_signbit;
       break;
 
     CASE_FLT_FN (CFN_BUILT_IN_ISINF):
+      if (gimple_call_num_args (call) != 1)
+	return;
       m_op1 = gimple_call_arg (call, 0);
       m_operator = &op_cfn_isinf;
       break;
 
     case CFN_BUILT_IN_ISFINITE:
+      if (gimple_call_num_args (call) != 1)
+	return;
       m_op1 = gimple_call_arg (call, 0);
       m_operator = &op_cfn_isfinite;
       break;
 
     case CFN_BUILT_IN_ISNORMAL:
+      if (gimple_call_num_args (call) != 1)
+	return;
       m_op1 = gimple_call_arg (call, 0);
       m_operator = &op_cfn_isnormal;
       break;
@@ -1565,7 +1575,9 @@ gimple_range_op_handler::maybe_builtin_c
     default:
       {
 	unsigned arg;
-	if (gimple_call_fnspec (call).returns_arg (&arg) && arg == 0)
+	if (gimple_call_fnspec (call).returns_arg (&arg)
+	    && arg == 0
+	    && gimple_call_num_args (call) > 0)
 	  {
 	    m_op1 = gimple_call_arg (call, 0);
 	    m_operator = &op_cfn_pass_through_arg1;
--- gcc/testsuite/gcc.dg/pr123431.c.jj	2026-01-09 15:10:58.084406597 +0100
+++ gcc/testsuite/gcc.dg/pr123431.c	2026-01-09 15:10:20.589057380 +0100
@@ -0,0 +1,19 @@
+/* PR tree-optimization/123431 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+extern void foo (int);
+
+extern inline __attribute__((always_inline)) void
+bar (int x, ...)
+{
+  if (__builtin_constant_p (__builtin_va_arg_pack ()))
+    foo (x);
+}
+
+void
+baz (int x)
+{
+  bar (1, 2);
+  bar (3, x);
+}