gh-156762: Fix tp_clear slot signature for operator.methodcaller

Lib/test/test_operator.py

@@ -2,6 +2,7 @@
 import inspect
 import pickle
 import sys
+import weakref
 from decimal import Decimal
 from fractions import Fraction
 
@@ -511,6 +512,21 @@ def return_arguments(self, *args, **kwds):
         f = operator.methodcaller('return_arguments', *many_positional_arguments, **many_kw_arguments)
         self.assertEqual(f(a), (many_positional_arguments, many_kw_arguments))
 
+    def test_methodcaller_cyclic_gc(self):
+        # gh-156762: Check for undefined behavior on calling methodcaller_clear()
+        operator = self.module
+
+        class C:
+            pass
+
+        c = C()
+        ref = weakref.ref(c)
+        c.m = operator.methodcaller('foo', c)
+        del c
+
+        support.gc_collect()
+        self.assertIsNone(ref())
+
     def test_inplace(self):
         operator = self.module
         class C(object):

Misc/NEWS.d/next/Core_and_Builtins/2026-09-13-10-43-24.gh-issue-156762.Kbf2mo.rst

@@ -0,0 +1,4 @@
+Fix undefined behaviour in :class:`operator.methodcaller`: its
+:c:member:`~PyTypeObject.tp_clear` slot function returned ``void`` instead of
+``int``, so the garbage collector called it through an incompatible function
+type. Patched by Shamil Abdulaev.

Modules/_operator.c

@@ -1740,7 +1740,7 @@ methodcaller_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     return (PyObject *)mc;
 }
 
-static void
+static int
 methodcaller_clear(PyObject *op)
 {
     methodcallerobject *mc = methodcallerobject_CAST(op);
@@ -1749,6 +1749,7 @@ methodcaller_clear(PyObject *op)
     Py_CLEAR(mc->kwds);
     Py_CLEAR(mc->vectorcall_args);
     Py_CLEAR(mc->vectorcall_kwnames);
+    return 0;
 }
 
 static void