I don't think there is. I think the code is functioning as expected — it's just written in a super confusing way. What should probably happen is that the compiler should throw an error about redefining a function in a super classes extension. I'm not a compiler engineer but a check for that seems like it would be intensive unless that information is codified in the AST somehow.
hmph. I agree, the compiler should probably not let you compile in that case. Funny enough this reminds me of "responder chain politics" in dynamically typed languages with duck typing.