[3.13] gh-131998: Fix NULL dereference when using an unbound method descriptor in a specialized code path (GH-132000)

Lib/test/test_types.py

@@ -6,6 +6,9 @@
     MISSING_C_DOCSTRINGS,
 )
 from test.test_import import no_rerun
+from test.support.script_helper import assert_python_ok
+from test.support.import_helper import import_fresh_module
+
 import collections.abc
 from collections import namedtuple, UserDict
 import copy
@@ -648,6 +651,24 @@ def test_traceback_and_frame_types(self):
     def test_capsule_type(self):
         self.assertIsInstance(_datetime.datetime_CAPI, types.CapsuleType)
 
+    def test_call_unbound_crash(self):
+        # GH-131998: The specialized instruction would get tricked into dereferencing
+        # a bound "self" that didn't exist if subsequently called unbound.
+        code = """if True:
+
+        def call(part):
+            [] + ([] + [])
+            part.pop()
+
+        for _ in range(3):
+            call(['a'])
+        try:
+            call(list)
+        except TypeError:
+            pass
+        """
+        assert_python_ok("-c", code)
+
 
 class UnionTests(unittest.TestCase):
 

Misc/NEWS.d/next/Core_and_Builtins/2025-04-01-22-24-19.gh-issue-131998.DvmZcT.rst

@@ -0,0 +1,2 @@
+Fix a crash when using an unbound method :term:`descriptor` object in a
+function where a bound method descriptor was used.

Python/bytecodes.c

@@ -3686,12 +3686,14 @@ dummy_func(
                 args--;
                 total_args++;
             }
+            DEOPT_IF(total_args == 0);
             PyMethodDescrObject *method = (PyMethodDescrObject *)callable;
             DEOPT_IF(!Py_IS_TYPE(method, &PyMethodDescr_Type));
             PyMethodDef *meth = method->d_method;
             DEOPT_IF(meth->ml_flags != (METH_FASTCALL|METH_KEYWORDS));
             PyTypeObject *d_type = method->d_common.d_type;
             PyObject *self = args[0];
+            assert(self != NULL);
             DEOPT_IF(!Py_IS_TYPE(self, d_type));
             STAT_INC(CALL, hit);
             int nargs = total_args - 1;
@@ -3754,11 +3756,13 @@ dummy_func(
                 total_args++;
             }
             PyMethodDescrObject *method = (PyMethodDescrObject *)callable;
+            DEOPT_IF(total_args == 0);
             /* Builtin METH_FASTCALL methods, without keywords */
             DEOPT_IF(!Py_IS_TYPE(method, &PyMethodDescr_Type));
             PyMethodDef *meth = method->d_method;
             DEOPT_IF(meth->ml_flags != METH_FASTCALL);
             PyObject *self = args[0];
+            assert(self != NULL);
             DEOPT_IF(!Py_IS_TYPE(self, method->d_common.d_type));
             STAT_INC(CALL, hit);
             PyCFunctionFast cfunc =