struct T { int x }
struct S { int x; int y }

int sum(S? s) {
    if?(T t = s) {
      return t.x;
    } else {
      return 0;
    }
}

int program (int argc, string[] argv) {
    var x = 0;
    x = x + sum(S null);
    x = x + sum(new S{x=4; y=5});
    return x;
}