Issue Description:

When declaring a nested type in a subclass that has the same name as a nested type in its superclass, the compiler doesn't warn about shadowing/overriding that name which could create unwanted ambiguity. I think the compiler should warn about that. Imagine the following code:

public class Foo {
    public enum Baz {}
}
public class Bar: Foo {
    public struct Baz {}
}

In that simple example the ambiguity is obvious. In a more complicated code base that could create serious problems. Having the compiler warn or even error at the declaration of Bar.Baz would help preventing that kind of problems.