dup field in sub-class should be reported by the compiler

[sa]

> Exactly what problem does this solve? If you're writing a derived class,
> and you create another field of the same name as a field in the base
> class, what bug has been created? The base class functions will still
> use the base class field, the derived class functions will still use the
> derived class field. The fields aren't polymorphic, so no hijacking can
> happen.
> I just don't see the issue.

It's less a problem if the programmer is *aware* of the re-definition; but in
practice, most the time, the programmer just forget, and will *assume* there's
only one definition, then it will cause strange error in his eye.

Now, even from an OO modelling point of view:

Dog {
  Tail tail;
}

MyDog {
  Tail tail;
}

Why should MyDog have a tail which is already defined in Dog?

It burden the programmer's thoughts, instead of simplify it.