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use core::pin::Pin;
use futures_core::future::{FusedFuture, Future, TryFuture};
use futures_core::task::{Context, Poll};
use pin_project::pin_project;

#[pin_project(project = TryFlattenErrProj)]
#[derive(Debug)]
pub enum TryFlattenErr<Fut1, Fut2> {
    First(#[pin] Fut1),
    Second(#[pin] Fut2),
    Empty,
}

impl<Fut1, Fut2> TryFlattenErr<Fut1, Fut2> {
    pub(crate) fn new(future: Fut1) -> Self {
        TryFlattenErr::First(future)
    }
}

impl<Fut> FusedFuture for TryFlattenErr<Fut, Fut::Error>
    where Fut: TryFuture,
          Fut::Error: TryFuture<Ok=Fut::Ok>,
{
    fn is_terminated(&self) -> bool {
        match self {
            TryFlattenErr::Empty => true,
            _ => false,
        }
    }
}

impl<Fut> Future for TryFlattenErr<Fut, Fut::Error>
    where Fut: TryFuture,
          Fut::Error: TryFuture<Ok=Fut::Ok>,
{
    type Output = Result<Fut::Ok, <Fut::Error as TryFuture>::Error>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        Poll::Ready(loop {
            match self.as_mut().project() {
                TryFlattenErrProj::First(f) => {
                    match ready!(f.try_poll(cx)) {
                        Err(f) => self.set(TryFlattenErr::Second(f)),
                        Ok(e) => {
                            self.set(TryFlattenErr::Empty);
                            break Ok(e);
                        }
                    }
                },
                TryFlattenErrProj::Second(f) => {
                    let output = ready!(f.try_poll(cx));
                    self.set(TryFlattenErr::Empty);
                    break output;
                },
                TryFlattenErrProj::Empty => panic!("TryFlattenErr polled after completion"),
            }
        })
    }
}