use std::future::Future;

pub trait X: Clone + Send + Unpin + Sync {
    fn x(&self) -> Box<dyn Future<Output = ()> + Unpin + Send>;
}

pub fn c<'a, X1: X + 'a>(x: &X1) -> Box<dyn Future<Output = ()> + Unpin + Send + 'a> {
    let x = x.clone();
    let res = x.x().then(move |_| x.x());
    Box::new(res)
}

impl X for &() {
    fn x(&self) -> Box<dyn Future<Output = ()> + Unpin + Send> {
        Box::new(Box::pin(async {}))
    }
}

fn main() {
    let _ = {
        let val = ();
        let reference = &val;
        c(&reference)
    };
}

/// ----

use std::pin::Pin;

trait FutureExt: Future {
    fn then<'a, F: Future + Send>(self, f: impl FnOnce(Self::Output) -> F + Send + 'a) -> Pin<Box<dyn Future<Output = F::Output> + Send + 'a>> where Self: Send + Sized + 'a, Self::Output: Send {
        Box::pin(async { f(self.await).await })
    }
}

impl<T: Future + ?Sized> FutureExt for T {}