Make decorators on special functions an error. Trac #649.

Cython/Compiler/Nodes.py

@@ -2056,7 +2056,7 @@ class DefNode(FuncDefNode):
     def analyse_signature(self, env):
         if self.entry.is_special:
             if self.decorators:
-                warning(self.pos, "special functions of cdef classes cannot have decorators", 1)
+                error(self.pos, "special functions of cdef classes cannot have decorators")
             self.entry.trivial_signature = len(self.args) == 1 and not (self.star_arg or self.starstar_arg)
         elif not env.directives['always_allow_keywords'] and not (self.star_arg or self.starstar_arg):
             # Use the simpler calling signature for zero- and one-argument functions.