aliases and templates

[Trass3r]

I already read about "Implicit Template Properties":
If a template has exactly one member in it, and the name of that member 
is the same as the template name, that member is assumed to be referred 
to in a template instantiation

I guess the following is something similar, but why are aliases used here?

template ParameterTypeTuple(dg)
{
    static if (is(dg P == function))
        alias P ParameterTypeTuple;
    else static if (is(dg P == delegate))
        alias ParameterTypeTuple!(P) ParameterTypeTuple;
    else static if (is(dg P == P*))
        alias ParameterTypeTuple!(P) ParameterTypeTuple;
    else
        static assert(0, "argument has no parameters");
}