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)
}