#![allow(dead_code, deprecated, unused_variables, unused_mut)]
#![feature(error_iter)]
use std::error::Error;
use std::fmt;
#[derive(Debug)]
struct A;
#[derive(Debug)]
struct B(Option<Box<dyn Error + 'static>>);
impl fmt::Display for A {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "A")
    }
}
impl fmt::Display for B {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "B")
    }
}
impl Error for A {}
impl Error for B {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        self.0.as_ref().map(|e| e.as_ref())
    }
}
fn main() {
    let b = B(Some(Box::new(A)));
    let err = &b as &dyn Error;
    let mut iter = err.sources();
    assert_eq!("B".to_string(), iter.next().unwrap().to_string());
    assert_eq!("A".to_string(), iter.next().unwrap().to_string());
    assert!(iter.next().is_none());
    assert!(iter.next().is_none());
}