type Foo = int | string

struct Struct {
    a ?Foo
}

fn r(a string) {}

fn process(s ?Foo) {
    mut t := Struct{
        a: s
    }
    if t.a is string {
        r(t.a)
    }
}

fn main() {
    s := ?Foo('hello')
    process(s)
}