alias this : cast turns into endless loop

[Namespace]

Trying with
        U opCast(U : Object)() {
                return new U();
        }
work fine. It isn't a solution, but i don't understand why this 
works and the following not:

        U opCast(U : Object)() {
                return cast(U) this._obj;
        }

I hope really some of you can explain that to me and help me to 
fix it.
And forgive me the old senseless thread title. ;)