I just discovered an alternative use of the `in`-operator

[Per Nordlöw via Digitalmars-d-learn]

I just discovered that the is-operator can be used as in

template ElementType(R)
{
    static if (is(typeof(R.init.front.init) T))
        alias ElementType = T;
    else
        alias ElementType = void;
}

. Very powerful. Is this documented?