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

[Walter Bright]

Robert Fraser wrote:
> I think the problem is with things like this:
> 
> class A
> {
>    string x;
>    string y;
> 
>    this()
>    {
>        x = "A.x";
>        y = "A.y";
>    }
> }
> 
> class B : A
> {
>    string x;
> 
>   this()
>    {
>        x = "B.x";
>    }
> }
> 
> public void main(string[] args)
> {
>    B b = new B();
>    A a = b; // a and b refer to the same object
>    writefln("A.x = %s", a.x);
>    writefln("B.x = %s", b.x);
>    writefln("A.y = %s", a.y);
>    writefln("B.y = %s", b.y);
> }
> 
> If A and B are maintained by different people, and the maintainer of B
 > adds a new field "y", the main code can fail silently, which is, as you
 > call it, "hijacking".


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.