fix declaration of Python classes with names that shadow builtins

Cython/Compiler/ParseTreeTransforms.py

@@ -1997,12 +1997,11 @@ class AlignFunctionDefinitions(CythonTransform):
         if pxd_def:
             if pxd_def.is_cclass:
                 return self.visit_CClassDefNode(node.as_cclass(), pxd_def)
-            else:
+            elif not pxd_def.scope or not pxd_def.scope.is_builtin_scope:
                 error(node.pos, "'%s' redeclared" % node.name)
                 if pxd_def.pos:
                     error(pxd_def.pos, "previous declaration here")
                 return None
-        else:
         return node
 
     def visit_CClassDefNode(self, node, pxd_def=None):

tests/run/class_redefine.py

+
+class set(object):
+    def __init__(self, x):
+        self.x = x
+
+SET = set([1])
+
+class set(object):
+    def __init__(self, x):
+        self.X = x
+
+def test_class_redef(x):
+    """
+    >>> SET.x
+    [1]
+    >>> test_class_redef(2).X
+    [2]
+    """
+    return set([x])