use std::sync::Arc;
trait A: AsA {}
trait B: A {}

trait AsA {
    fn as_a(&self) -> &A;
}

impl<T: A> AsA for T {
    fn as_a(&self) -> &A {
        self
    }
}

struct Y;
impl A for Y {}
impl B for Y {}

fn another(x: &dyn A){ }

fn something(x: Arc<dyn B>) {
    another(x.as_a());
}

fn main() {
    let y = Y;
    something(Arc::new(y));
}