Cast Object - get null

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> one another, you're going to need to overload opCast.
>
> - Jonathan M Davis

Thanks for your answer.
I have tried to overload opCast in this way:
U opCast(U)() const {
     return U(this.x, this.y);
}

But then i get this compiler error
cast.d(36): Error: no property 'opCall' for type 'object.Object'
cast.d(299): Error: template instance 
cast.Vector2D!(short).Vector2D.opCast!(Obj
ect) error instantiating
cast.d(36): Error: no property 'opCall' for type 'object.Object'
cast.d(299): Error: template instance 
cast.Vector2D!(float).Vector2D.opCast!(Obj
ect) error instantiating

The dokumentation of opCast is very short, did i understand 
opCast wrong?