Index: Lib/abc.py
===================================================================
--- Lib/abc.py	(revision 79493)
+++ Lib/abc.py	(working copy)
@@ -3,6 +3,7 @@
 
 """Abstract Base Classes (ABCs) according to PEP 3119."""
 
+from _weakrefset import WeakSet
 
 # Instance of old-style class
 class _C: pass
@@ -93,9 +94,9 @@
                     abstracts.add(name)
         cls.__abstractmethods__ = frozenset(abstracts)
         # Set up inheritance registry
-        cls._abc_registry = set()
-        cls._abc_cache = set()
-        cls._abc_negative_cache = set()
+        cls._abc_registry = WeakSet()
+        cls._abc_cache = WeakSet()
+        cls._abc_negative_cache = WeakSet()
         cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
         return cls
 
@@ -150,7 +151,7 @@
         # Check negative cache; may have to invalidate
         if cls._abc_negative_cache_version < ABCMeta._abc_invalidation_counter:
             # Invalidate the negative cache
-            cls._abc_negative_cache = set()
+            cls._abc_negative_cache = WeakSet()
             cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
         elif subclass in cls._abc_negative_cache:
             return False
Index: Lib/test/test_collections.py
===================================================================
--- Lib/test/test_collections.py	(revision 79493)
+++ Lib/test/test_collections.py	(working copy)
@@ -1,5 +1,5 @@
 
-import unittest, doctest
+import unittest, doctest, gc
 import inspect
 from test import test_support
 from collections import namedtuple, Counter, OrderedDict
@@ -454,6 +454,36 @@
         self.validate_abstract_methods(MutableSet, '__contains__', '__iter__', '__len__',
             'add', 'discard')
 
+    def test_cache_leak(self):
+        class sortedlist(collections.Sequence):
+            def __init__(self, v):
+                self.add(v)
+
+            def add(self, value):
+                pass
+
+            def __len__(self):
+                pass
+
+            def __getitem__(self, index):
+                pass
+
+        class sortedset(sortedlist):
+            def add(self, value):
+                sortedlist.add(self, value)
+
+        def leak():
+            class T(sortedset):
+                pass
+            T(1)
+
+        count = []
+        for i in range(3):
+            leak()
+            gc.collect()
+            count.append(len(gc.get_objects()))
+        self.assertEqual(count[1], count[2])
+
     def test_issue_5647(self):
         # MutableSet.__iand__ mutated the set during iteration
         s = WithSet('abcd')
Index: Modules/Setup.dist
===================================================================
--- Modules/Setup.dist	(revision 79493)
+++ Modules/Setup.dist	(working copy)
@@ -118,6 +118,7 @@
 				# if $HOME is not set
 _sre _sre.c			# Fredrik Lundh's new regular expressions
 _codecs _codecsmodule.c		# access to the builtin codecs and codec registry
+_weakref _weakref.c             # weak references
 
 # The zipimport module is always imported at startup. Having it as a
 # builtin module avoids some bootstrapping problems and reduces overhead.