Issue Description:

I have

final public class A<T> {
    public struct AB {
    }
}
final public class C<T>: A<T> {
}

I'm trying to extend it:

extension C.AB {
}  

But this gives a confusing error

Constrained extension must be declared on the unspecialized generic type 'AB' with constraints specified by a 'where' clause

The trick to making it work is

extension A.AB {
} 

Though theoretically that allows using the extension with A as well, which might be semantically wrong and confusing to users in some situations.

In any event, I mainly think that, if at the compiler level there is no need or no ability to distinguish between A.AB and C.AB (actually "inheriting" a nested type as a distinctly referenceable entity would be the ideal case), at the least I would want it to present a more helpful error message like "No nested type AB in C, did you perhaps mean A.AB from the superclass?".