trait TraitA { type Item: ?Sized; }

trait TraitB<T> {
    fn hello(&self);
}

impl<X: TraitA> TraitB<X> for X::Item {
    fn hello(&self) {
        println!("Hello for X::Item");
    }
}

impl TraitB<()> for () {
    fn hello(&self) {
        println!("Hello for ()");
    }
}

impl TraitA for () {
    type Item = dyn TraitB<()>;
}

fn call_hello<T: ?Sized + TraitB<()>>(t: &T) {
    t.hello();
}

fn main() {
    call_hello::<()>(&());
    call_hello::<dyn TraitB<()>>(&() as &dyn TraitB<()>);
}