gh-159739: Keep the instance type alive during ABC subclass checks

Lib/test/test_abc.py

@@ -7,11 +7,12 @@
 """Unit tests for abc.py."""
 
 import unittest
+import weakref
 
 import abc
 import _py_abc
 from inspect import isabstract
-from test.support import warnings_helper
+from test.support import gc_collect, warnings_helper
 
 def test_factory(abc_ABCMeta, abc_get_cache_token):
     class TestLegacyAPI(unittest.TestCase):
@@ -380,6 +381,33 @@ class C(str): pass
             self.assertIsSubclass(C, A)
             self.assertIsSubclass(C, (A,))
 
+        def test_instancecheck_class_changes_in_subclasscheck(self):
+            class Original:
+                def __getattribute__(self, name):
+                    if name == '__class__':
+                        return int
+                    return object.__getattribute__(self, name)
+
+            class Replacement:
+                pass
+
+            instance = Original()
+            original = weakref.ref(Original)
+            del Original
+
+            class Meta(abc_ABCMeta):
+                def __subclasscheck__(cls, candidate):
+                    if candidate is int:
+                        instance.__class__ = Replacement
+                        gc_collect()
+                        self.assertIsNotNone(original())
+                    return False
+
+            class Interface(metaclass=Meta):
+                pass
+
+            self.assertNotIsInstance(instance, Interface)
+
         def test_instancecheck_no_class(self):
             # gh-153772: __instancecheck__ must fall back to type(instance)
             # when the instance has no __class__, matching isinstance().

Misc/NEWS.d/next/Library/2026-10-21-21-26-10.gh-issue-159739.yBc6Ei.rst

@@ -0,0 +1 @@
+Fix a possible crash in :class:`abc.ABCMeta` instance checks when a subclass-checking callback changes the instance’s class.

Modules/_abc.c

@@ -664,9 +664,12 @@ _abc__abc_instancecheck_impl(PyObject *module, PyObject *self,
                                            subclass);
         goto end;
     }
+    /* A subclass check can change the instance's actual type. */
+    Py_INCREF(subtype);
     result = PyObject_CallMethodOneArg(self, &_Py_ID(__subclasscheck__),
                                        subclass);
     if (result == NULL) {
+        Py_DECREF(subtype);
         goto end;
     }
 
@@ -684,6 +687,7 @@ _abc__abc_instancecheck_impl(PyObject *module, PyObject *self,
     default:
         Py_UNREACHABLE();
     }
+    Py_DECREF(subtype);
 
 end:
     Py_XDECREF(impl);