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

On Wednesday, 13 June 2018 at 09:41:45 UTC, Luís Marques wrote:

> ...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))

U needs to be alias U, since S is not a type, but a template. This works:

template X(T)
{
    static if(is(T t == U!n, alias 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));

Also, you might want to consider the Learn forum for these kinds of questions in the future. :)

--
  Simen

On Wednesday, 13 June 2018 at 10:34:48 UTC, Simen Kjærås wrote:
> Also, you might want to consider the Learn forum for these kinds of questions in the future. :)

You're right. What happened is that I went to check the syntax in the "D Templates: A Tutorial" PDF, and it has some material on how some possible uses of is expressions used to not be accepted by the frontend, so I though this might be a similar case: perhaps a bug or an unsupported scenario. A bit arrogant on my part, but I do stumble into bugs from time to time, so it was plausible :-). And if it was a bug/limitation then arguably this was the better forum.