gh-113157 gh-89519: Fix method descriptors (gh-113233)

Restore behaviors before classmethod descriptor chaining was introduced.

Author: rhettinger

Lib/test/test_descr.py

@@ -5004,6 +5004,21 @@ class Child(Parent):
         gc.collect()
         self.assertEqual(Parent.__subclasses__(), [])
 
+    def test_instance_method_get_behavior(self):
+        # test case for gh-113157
+
+        class A:
+            def meth(self):
+                return self
+
+        class B:
+            pass
+
+        a = A()
+        b = B()
+        b.meth = a.meth.__get__(b, B)
+        self.assertEqual(b.meth(), a)
+
     def test_attr_raise_through_property(self):
         # test case for gh-103272
         class A:

Objects/classobject.c

@@ -319,6 +319,13 @@ method_traverse(PyMethodObject *im, visitproc visit, void *arg)
     return 0;
 }
 
+static PyObject *
+method_descr_get(PyObject *meth, PyObject *obj, PyObject *cls)
+{
+    Py_INCREF(meth);
+    return meth;
+}
+
 PyTypeObject PyMethod_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
     .tp_name = "method",
@@ -339,6 +346,7 @@ PyTypeObject PyMethod_Type = {
     .tp_methods = method_methods,
     .tp_members = method_memberlist,
     .tp_getset = method_getset,
+    .tp_descr_get = method_descr_get,
     .tp_new = method_new,
 };