is(T t == U!n, U, int n)

[Luís Marques via Digitalmars-d]

If this works:

    template X(T)
    {
        static if(is(T t == S!n, int n))
        {
            static if(n == 0)
                alias X = AliasSeq!();
            else
                alias X = S!0;
        }
        else
        {
            static assert(0);
        }
    }

    class S(int n) { }

    pragma(msg, X!(S!3)); // S!0
    pragma(msg, X!(S!0)); // ()

...why can't I generalize it to match U!n, for some U, int n?

    template X(T)
    {
        static if(is(T t == U!n, U, int n))
        {
            static if(n == 0)
                alias X = AliasSeq!();
            else
                alias X = U!0;
        }
        else
        {
            static assert(0);
        }
    }

    class S(int n) { }

    pragma(msg, X!(S!3)); // static assert: 0