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

Lib/test/test_types.py

@@ -5,6 +5,8 @@
     MISSING_C_DOCSTRINGS,
 )
 from test.test_import import no_rerun
+from test.support.script_helper import assert_python_ok
+
 import collections.abc
 from collections import namedtuple
 import copy
@@ -643,6 +645,24 @@ def test_traceback_and_frame_types(self):
         self.assertIsInstance(exc.__traceback__, types.TracebackType)
         self.assertIsInstance(exc.__traceback__.tb_frame, types.FrameType)
 
+    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

@@ -3108,13 +3108,15 @@ dummy_func(
                 args--;
                 total_args++;
             }
+            DEOPT_IF(total_args == 0, CALL);
             PyMethodDescrObject *callable =
                 (PyMethodDescrObject *)PEEK(total_args + 1);
             DEOPT_IF(!Py_IS_TYPE(callable, &PyMethodDescr_Type), CALL);
             PyMethodDef *meth = callable->d_method;
             DEOPT_IF(meth->ml_flags != (METH_FASTCALL|METH_KEYWORDS), CALL);
             PyTypeObject *d_type = callable->d_common.d_type;
             PyObject *self = args[0];
+            assert(self != NULL);
             DEOPT_IF(!Py_IS_TYPE(self, d_type), CALL);
             STAT_INC(CALL, hit);
             int nargs = total_args - 1;
@@ -3173,13 +3175,15 @@ dummy_func(
                 args--;
                 total_args++;
             }
+            DEOPT_IF(total_args == 0, CALL);
             PyMethodDescrObject *callable =
                 (PyMethodDescrObject *)PEEK(total_args + 1);
             /* Builtin METH_FASTCALL methods, without keywords */
             DEOPT_IF(!Py_IS_TYPE(callable, &PyMethodDescr_Type), CALL);
             PyMethodDef *meth = callable->d_method;
             DEOPT_IF(meth->ml_flags != METH_FASTCALL, CALL);
             PyObject *self = args[0];
+            assert(self != NULL);
             DEOPT_IF(!Py_IS_TYPE(self, callable->d_common.d_type), CALL);
             STAT_INC(CALL, hit);
             _PyCFunctionFast cfunc =