bpo-36922: use Py_TPFLAGS_METHOD_DESCRIPTOR in lookup_maybe_method() (GH-13865)

(cherry picked from commit 2e9954d)

Co-authored-by: Jeroen Demeyer <J.Demeyer@UGent.be>

Author: miss-islington

Lib/test/test_descr.py

@@ -4647,9 +4647,11 @@ class Foo:
     def test_mixing_slot_wrappers(self):
         class X(dict):
             __setattr__ = dict.__setitem__
+            __neg__ = dict.copy
         x = X()
         x.y = 42
         self.assertEqual(x["y"], 42)
+        self.assertEqual(x, -x)
 
     def test_slot_shadows_class_variable(self):
         with self.assertRaises(ValueError) as cm:

Misc/NEWS.d/next/Core and Builtins/2019-06-06-13-59-52.bpo-36922.EMZ3TF.rst

@@ -0,0 +1 @@
+Slot functions optimize any callable with ``Py_TPFLAGS_METHOD_DESCRIPTOR`` instead of only instances of ``function``.

Objects/typeobject.c

@@ -1412,7 +1412,7 @@ lookup_maybe_method(PyObject *self, _Py_Identifier *attrid, int *unbound)
         return NULL;
     }
 
-    if (PyFunction_Check(res)) {
+    if (PyType_HasFeature(Py_TYPE(res), Py_TPFLAGS_METHOD_DESCRIPTOR)) {
         /* Avoid temporary PyMethodObject */
         *unbound = 1;
         Py_INCREF(res);