#![allow(dead_code, deprecated, unused_variables, unused_mut)]
#![feature(error_generic_member_access)]
use core::error::Request;
use core::error::request_value;
#[derive(Debug)]
struct Parent(Option<u8>);
impl std::fmt::Display for Parent {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "a parent failed")
    }
}
impl std::error::Error for Parent {
    fn provide<'a>(&'a self, request: &mut Request<'a>) {
        if let Some(v) = self.0 {
            request.provide_value::<u8>(v);
        }
    }
}
#[derive(Debug)]
struct Child {
    parent: Parent,
}
impl Child {
    fn an_expensive_computation(&self) -> Option<u8> {
        Some(99)
    }
}
impl std::fmt::Display for Child {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "child failed: \n  because of parent: {}", self.parent)
    }
}
impl std::error::Error for Child {
    fn provide<'a>(&'a self, request: &mut Request<'a>) {
        self.parent.provide(request);
        if request.would_be_satisfied_by_value_of::<u8>() {
            if let Some(v) = self.an_expensive_computation() {
                request.provide_value::<u8>(v);
            }
        }
        assert!(! request.would_be_satisfied_by_value_of::< u8 > ());
    }
}
fn main() {
    let parent = Parent(Some(42));
    let child = Child { parent };
    assert_eq!(Some(42), request_value::< u8 > (& child));
    let parent = Parent(None);
    let child = Child { parent };
    assert_eq!(Some(99), request_value::< u8 > (& child));
}