better error messages during constant folding, forward constants also through type coercion

Cython/Compiler/ExprNodes.py

@@ -27,7 +27,8 @@ try:
 except NameError:
     from sets import Set as set
 
-not_a_constant = object()
+class NotConstant(object): pass # just for the name
+not_a_constant = NotConstant()
 constant_value_not_set = object()
 
 class ExprNode(Node):
@@ -4840,6 +4841,9 @@ class CoercionNode(NewTempExprNode):
         if debug_coercion:
             print("%s Coercing %s" % (self, self.arg))
 
+    def calculate_constant_result(self):
+        self.constant_result = self.arg.constant_result
+            
     def annotate(self, code):
         self.arg.annotate(code)
         if self.arg.type != self.type: