Index: Objects/abstract.c
===================================================================
--- Objects/abstract.c	(revision 64559)
+++ Objects/abstract.c	(working copy)
@@ -2915,6 +2915,24 @@
 	checker = PyObject_GetAttr(cls, name);
 	if (checker == NULL && PyErr_Occurred())
 		PyErr_Clear();
+
+	/* Ignore the checker if it comes as an unbound method: 
+	 * we call it with 'inst' as first argument, which is wrong 
+	 * anyway, and leads to an infinite recursion because of the 
+	 * isinstance(self, cls) check.
+	 * This may happen however when the class is actually a
+	 * metaclass, as in 
+	 *      isintance(x, abc.ABCMeta)
+	 */
+	if (checker && PyObject_TypeCheck(checker, &PyMethod_Type) &&
+		PyMethod_GET_SELF(checker) == NULL) {
+		Py_DECREF(checker);
+		checker = PyObject_GetAttr((PyObject*)Py_TYPE(cls->ob_type), name);
+		if (checker == NULL && PyErr_Occurred())
+			PyErr_Clear();
+		Py_XINCREF(checker);
+	}
+
 	if (checker != NULL) {
 		PyObject *res;
 		int ok = -1;
Index: Lib/test/test_typechecks.py
===================================================================
--- Lib/test/test_typechecks.py	(revision 64559)
+++ Lib/test/test_typechecks.py	(working copy)
@@ -29,10 +29,6 @@
     pass
 
 
-class Evil:
-    def __instancecheck__(self, inst): return False
-
-
 class TypeChecksTest(unittest.TestCase):
 
     def testIsSubclassInternal(self):
@@ -62,13 +58,23 @@
         self.assertEqual(isinstance(SubInt(), SubInt), True)
         self.assertEqual(isinstance(42, SubInt), False)
 
-    def testInfiniteRecursionCaughtProperly(self):
-        e = Evil()
-        # This invokes isinstance() recursively, until the stack is exhausted.
-        self.assertRaises(RuntimeError, isinstance, e, Evil)
-        # XXX How to check the same situation for issubclass()?
+    def testUnboundCheckMethod(self):
+        class Class:
+            # Not a @classmethod: __instancecheck__ is not used
+            def __instancecheck__(self, inst):
+                raise ValueError("test failed")
 
+        self.assertEqual(isinstance(3, Class), False)
 
+        class SubInt2(Integer):
+            # Not a @classmethod: __instancecheck__ is not used
+            # its metaclass.__instancecheck__ is used
+            def __instancecheck__(self, inst):
+                raise ValueError("test failed")
+
+        self.assertEqual(isinstance(3, SubInt2), False)
+        self.assertEqual(isinstance(SubInt2(), SubInt2), True)
+
 def test_main():
     test_support.run_unittest(TypeChecksTest)