diff -r a3c9c2c92b87 Lib/test/test_descr.py
--- a/Lib/test/test_descr.py	Fri Mar 20 20:04:21 2015 +0200
+++ b/Lib/test/test_descr.py	Fri Mar 20 16:21:27 2015 -0400
@@ -3020,8 +3020,6 @@
         cant(object(), list)
         cant(list(), object)
         class Int(int): __slots__ = []
-        cant(2, Int)
-        cant(Int(), int)
         cant(True, int)
         cant(2, bool)
         o = object()
diff -r a3c9c2c92b87 Lib/test/test_gc.py
--- a/Lib/test/test_gc.py	Fri Mar 20 20:04:21 2015 +0200
+++ b/Lib/test/test_gc.py	Fri Mar 20 16:21:27 2015 -0400
@@ -3,6 +3,7 @@
                             strip_python_stderr, cpython_only)
 from test.script_helper import assert_python_ok, make_script, temp_dir
 
+import array
 import sys
 import time
 import gc
@@ -550,11 +551,28 @@
 
         class UserClass:
             pass
+
+        class UserInt(int):
+            pass
+
+        class UserClassSlots:
+            __slots__ = ()
+
+        class UserIntSlots(int):
+            __slots__ = ()
+
+        class UserArraySlots(array.array):
+            __slots__ = ()
+
         self.assertTrue(gc.is_tracked(gc))
         self.assertTrue(gc.is_tracked(UserClass))
         self.assertTrue(gc.is_tracked(UserClass()))
+        self.assertTrue(gc.is_tracked(UserInt()))
         self.assertTrue(gc.is_tracked([]))
         self.assertTrue(gc.is_tracked(set()))
+        self.assertFalse(gc.is_tracked(UserClassSlots()))
+        self.assertFalse(gc.is_tracked(UserIntSlots()))
+        self.assertFalse(gc.is_tracked(UserArraySlots('i')))
 
     def test_bug1055820b(self):
         # Corresponds to temp2b.py in the bug report.
diff -r a3c9c2c92b87 Objects/typeobject.c
--- a/Objects/typeobject.c	Fri Mar 20 20:04:21 2015 +0200
+++ b/Objects/typeobject.c	Fri Mar 20 16:21:27 2015 -0400
@@ -2645,9 +2645,10 @@
     }
     type->tp_dealloc = subtype_dealloc;
 
-    /* Enable GC unless there are really no instance variables possible */
-    if (!(type->tp_basicsize == sizeof(PyObject) &&
-          type->tp_itemsize == 0))
+    /* Enable GC unless this class is not adding new instance variables and
+       the base class did not use GC. */
+    if ((base->tp_flags & Py_TPFLAGS_HAVE_GC) ||
+        type->tp_basicsize > base->tp_basicsize)
         type->tp_flags |= Py_TPFLAGS_HAVE_GC;
 
     /* Always override allocation strategy to use regular heap */