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

[Luís Marques]

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