cast question

[Dan]

Why do the commented out calls to goo fail?

Thanks
Dan

-------------------------------------
import std.stdio;
struct S {
}
void goo(const ref S s) {
  writeln(s);
}
struct T {
  S s;
  void foo(const ref T other) const {
    goo(s);
    // Error: function fdsaf.goo (ref const(S) s) is
    //        not callable using argument types (S)
    // goo(cast(S)s);
    // goo(cast()s);   // same error
    writeln(typeid(S), " ", typeid(s), " ",
            typeid(cast()s), " ", typeid(cast(S)s));

  }
}
void main() {
  T t;
  t.foo(t);
  const S s1;
  S s2;
  goo(s1);
  goo(s2);
  goo(cast(S) s2);
  // goo(cast(S) s1); Also fails
}