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

[Robert Fraser]

Walter Bright Wrote:

> It's not hijacking at all, it's overriding. a.y does not change, i.e. it 
> is not hijacked by adding a definition to B. Only b.y changes, and 
> that's unsurprising.
> 

Ah, I see, I misunderstood whatb you meant by hijacking. That said, if it's not too hard, it seems to be an important issue for people... so could we at least have a warning?