gh-131998: Fix NULL dereference when using an unbound method descriptor in a specialized code path

Lib/test/test_types.py

@@ -4,6 +4,7 @@
     run_with_locale, cpython_only, no_rerun,
     MISSING_C_DOCSTRINGS,
 )
+from test.support.script_helper import assert_python_ok
 import collections.abc
 from collections import namedtuple, UserDict
 import copy
@@ -646,6 +647,22 @@ def test_traceback_and_frame_types(self):
     def test_capsule_type(self):
         self.assertIsInstance(_datetime.datetime_CAPI, types.CapsuleType)
 
+    def test_gh131998(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:
+        import glob
+        def call(part):
+            part.pop()
+
+        try:
+            call(['a'])
+            call(list)
+        except:
+            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

@@ -4304,6 +4304,7 @@ dummy_func(
             EXIT_IF(meth->ml_flags != (METH_FASTCALL|METH_KEYWORDS));
             PyTypeObject *d_type = method->d_common.d_type;
             PyObject *self = PyStackRef_AsPyObjectBorrow(arguments[0]);
+            EXIT_IF(self == NULL);
             EXIT_IF(!Py_IS_TYPE(self, d_type));
             STAT_INC(CALL, hit);
             int nargs = total_args - 1;
@@ -4382,6 +4383,7 @@ dummy_func(
             PyMethodDef *meth = method->d_method;
             EXIT_IF(meth->ml_flags != METH_FASTCALL);
             PyObject *self = PyStackRef_AsPyObjectBorrow(arguments[0]);
+            EXIT_IF(self == NULL);
             EXIT_IF(!Py_IS_TYPE(self, method->d_common.d_type));
             STAT_INC(CALL, hit);
             int nargs = total_args - 1;