fix infinite recursion in CF based None-check removal

Cython/Compiler/ExprNodes.py

@@ -1483,12 +1483,22 @@ class NameNode(AtomicExprNode):
 
     def may_be_none(self):
         if self.cf_state:
+            # gard against infinite recursion on self-dependencies
+            if getattr(self, '_none_checking', False):
+                # self-dependency - either this node receives a None
+                # value from *another* node, or it can not reference
+                # None at this point => safe to assume "not None"
+                return False
+            self._none_checking = True
             # evaluate control flow state to see if there were any
             # potential None values assigned to the node so far
+            may_be_none = False
             for assignment in self.cf_state:
                 if assignment.rhs.may_be_none():
-                    return True
-            return False
+                    may_be_none = True
+                    break
+            del self._none_checking
+            return may_be_none
         return super(NameNode, self).may_be_none()
 
     def nonlocally_immutable(self):

tests/run/cf_none.pyx

@@ -82,3 +82,34 @@ def conditional_not_none(a, dict d not None):
     if a:
         d = {}
     return d.get(1)
+
+@cython.test_fail_if_path_exists('//NoneCheckNode')
+def self_dependency(int x):
+    """
+    >>> self_dependency(1)
+    (1, 2)
+    >>> self_dependency(2)
+    (None, None)
+    """
+    cdef dict a, b
+    a = {1:2}
+    b = {2:1}
+    for i in range(x):
+        a,b = b,a
+    return a.get(2), b.get(1)
+
+@cython.test_assert_path_exists('//NoneCheckNode')
+def self_dependency_none(int x):
+    """
+    >>> self_dependency_none(False)
+    1
+    >>> self_dependency_none(True)
+    Traceback (most recent call last):
+    AttributeError: 'NoneType' object has no attribute 'get'
+    """
+    cdef dict a, b
+    a = None
+    b = {2:1}
+    if x:
+        a,b = b,a
+    return b.get(2)