Hiding mixin template members should be allowed.

[Timon Gehr]

Oh, I see, the problem is that the functions in the mixin would call the hidden
functions instead of the new ones.

I have also found the workaround:

class DefaultTalkyDuck: Duck{
    mixin DuckImpl;
}
class TalkyDuck: DefaultTalkyDuck{
    override void quack(){writeln("quack! quack!");}
}

Sorry for the noise.

-Timon