Safer casts

[Robert Fraser]

Robert Fraser wrote:
>  [...]

Also, I find the cast-returns-null-on-failure behavior quite useful. In 
your system, this piece of code:

Shape s = getShape();
if(auto r = cast(Rectangle) s) {
     return r.length * r.width;
}
else if(auto c = cast(Circle) s) {
     return PI * c.radius * c.radius;
}

... would become ...

Shape s = getShape();
if(is!(Rectangle)(s)) {
     auto r = class!(Rectangle) s;
     return r.length * r.width;
}
else if(is!(Rectangle)(s)) {
     auto c = class!(Circle) s;
     return PI * c.radius * c.radius;
}

Which is more typing and uglier IMO. Since if you want this behavior, 
you can already make templates to do it, why force it down people's throats?