defered new feature

[BCS]

A number of times I have found my self wanting to have "new C(args)" return 
a class derived from C. I know this can be done with a static function or 
the like but syntactically, it's unappealing.

My suggestion is that, with some kind of flag, constructors be allowed and 
required to construct an object and return it.

One use cases:

> class C
> {
>    abstract void F(); /// abstract class
>    abstract this(int i) /// "abstract" this is just one idea, others would 
work as well
>    {
>        if(i < 0)
>           return new N(-i);
>        else
>           return new P(i);
>    }
> }
>
> private class N : C
> {
>    void F();
>    this(uint i){}
> }
>
> private class P : C
> {
>    void F();
>    this(uint i){}
> }