is expression on templated types

[simendsjo]

I have a templated struct, and I'd like to check if a given type is this 
struct. I have no idea how I should write the is expression..

struct S(int C, int R, T) {
    T[C][R] data;
}
template isS(T) {
    enum isS = is(T : S); // How can I see if T is a kind of S no 
matter what values S is parameterized on?
}
unittest {
    static assert(isS!(S!(1,1,float)));
    static assert(!isS!(float[1][1]));
}