use std::sync::Arc;

trait A {
    fn as_b(&self) -> Option<&B> { None }
}

#[derive(Default)]
struct B;
impl A for B {
    fn as_b(&self) -> Option<&B> { Some(self) }
}

fn main() {
    let obj: Arc<A> = Arc::new(B::default());
    let casted = obj.as_b();
    assert!(casted.is_some());
}