Can't use is expression on nested type

[Emma]

Hello, I was doing some template nonsense and came across this 
weird issue. This compiles fine:

```d
struct Foo(string S) {}

bool foo(T)(T it) {
     static if (is(T == Foo!S, string S)) {
         return true;
     } else {
         return false;
     }
}

void main() {
     foo(Foo!"x"());
}
```

but the below fails with a “undefined identifier 'S'” error!

```d
struct X {
     struct Foo(string S) {}
}

bool foo(T)(T it) {
     static if (is(T == X.Foo!S, string S)) {
         return true;
     } else {
         return false;
     }
}

void main() {
     foo(X.Foo!"x"());
}
```

Am I just doing something obviously wrong, or is this a compiler 
bug? If so, is there a workaround? Thank you.