Check whether a type is a instantiated by a template struct

[Henning Pohl]

So the struct is defined as:

struct S(T) {
}
template isS(T) {
    // ...
}

static assert(isS(S!float));
static assert(!isS(float));

There may be some nasty ways using fullQualifiedName!T and so on 
but I guess there is a better way, isn't it?