#![feature(
    async_await,
    async_closure,
)]

use core::future::Future;

async fn closure_caller<F>(f: Box<F>) -> Result<usize, ()>
    where F: FnOnce(i64) -> dyn Future<Output = Result<usize, ()>>,
{
    f(7).await
}

fn main() {
    closure_caller(
        Box::new(async move |n| {
            Ok(n + 5)
        })
    );
}