Inexplicably, recurse_down_subclasses() was comparing the object
gotten from a weak reference to NULL instead of to None. This caused
the following assert() to fail (but only in 2.2 in the debug build --
I have to find a better test case). Will backport.
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index b9890ea..0051179 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -4029,7 +4029,8 @@
ref = PyList_GET_ITEM(subclasses, i);
assert(PyWeakref_CheckRef(ref));
subclass = (PyTypeObject *)PyWeakref_GET_OBJECT(ref);
- if (subclass == NULL)
+ assert(subclass != NULL);
+ if ((PyObject *)subclass == Py_None)
continue;
assert(PyType_Check(subclass));
/* Avoid recursing down into unaffected classes */