In this section https://docs.python.org/3/howto/descriptor.html#class-methods in the example of python implementation of ClassMethod inside __get__ method I think that we should check hasattr(self.f, "__get__") instead of hasattr(obj, "__get__").

It looks like it should be:

    if hasattr(type(self.f), "__get__"):


Here's the relevant C code:

static PyObject *
cm_descr_get(PyObject *self, PyObject *obj, PyObject *type)
{
    classmethod *cm = (classmethod *)self;

    if (cm->cm_callable == NULL) {
        PyErr_SetString(PyExc_RuntimeError,
                        "uninitialized classmethod object");
        return NULL;
    }
    if (type == NULL)
        type = (PyObject *)(Py_TYPE(obj));
    if (Py_TYPE(cm->cm_callable)->tp_descr_get != NULL) {
        return Py_TYPE(cm->cm_callable)->tp_descr_get(cm->cm_callable, type,
                                                      NULL);
    }
    return PyMethod_New(cm->cm_callable, type);
}

New changeset 14092b5a4ae4caf1c77f685450016a0d1ad0bd6c by Raymond Hettinger in branch 'master':
bpo-43917: Fix pure python equivalent for classmethod (GH-25544)
https://github.com/python/cpython/commit/14092b5a4ae4caf1c77f685450016a0d1ad0bd6c

New changeset 34be48450f03b121be10a9f8e8989603478f0469 by Miss Islington (bot) in branch '3.9':
bpo-43917: Fix pure python equivalent for classmethod (GH-25544) (GH-25546)
https://github.com/python/cpython/commit/34be48450f03b121be10a9f8e8989603478f0469

Okay, it's fixed.  Thanks for the report.