Capture and document the caveat about metaclass superclass

documentation/index.rst

@@ -359,6 +359,12 @@ Python 2 and 3.
            pass
        MyClass = add_metaclass(Meta)(MyClass)
 
+   This technique works by re-building MyClass from the indicated metaclass,
+   but in the rare case where the parent has a metaclass that is a
+   superclass of the metaclass used with the decorator, this approach will
+   fail with a TypeError. See `issue 127
+   <https://github.com/benjaminp/six/issues/127>`_ for details.
+
 
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test_six.py

@@ -895,6 +895,30 @@ class B: pass
     assert A.B.__qualname__ == expected
 
 
+def test_add_metaclass_child_meta():
+    """
+    Capture the known failure in the rare case where the parent
+    has a metaclass that is a superclass of the metaclass used
+    with the decorator (#127).
+    """
+    class MetaA(type): pass
+    class MetaB(type): pass
+
+    class A(object): pass
+    A = six.add_metaclass(MetaA)(A)
+
+    class B(object): pass
+    B = six.add_metaclass(MetaB)(B)
+
+    class MetaC(MetaA, MetaB): pass
+
+    with py.test.raises(TypeError) as exc:
+        class C(A, B): pass
+        C = six.add_metaclass(MetaC)(C)
+
+    assert 'metaclass conflict' in str(exc.value)
+
+
 @py.test.mark.skipif("sys.version_info[:2] < (2, 7) or sys.version_info[:2] in ((3, 0), (3, 1))")
 def test_assertCountEqual():
     class TestAssertCountEqual(unittest.TestCase):