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    use {Future, Poll, Async};
    use future::Either;

    /// Future for the `select2` combinator, waiting for one of two differently-typed
    /// futures to complete.
    ///
    /// This is created by the [`Future::select2`] method.
    ///
    /// [`Future::select2`]: trait.Future.html#method.select2
    #[must_use = "futures do nothing unless polled"]
    #[derive(Debug)]
    pub struct Select2<A, B> {
        inner: Option<(A, B)>,
    }

    pub fn new<A, B>(a: A, b: B) -> Select2<A, B> {
        Select2 { inner: Some((a, b)) }
    }

    impl<A, B> Future for Select2<A, B> where A: Future, B: Future {
        type Item = Either<(A::Item, B), (B::Item, A)>;
        type Error = Either<(A::Error, B), (B::Error, A)>;

        fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
            let (mut a, mut b) = self.inner.take().expect("cannot poll Select2 twice");
            match a.poll() {
                Err(e) => Err(Either::A((e, b))),
                Ok(Async::Ready(x)) => Ok(Async::Ready(Either::A((x, b)))),
                Ok(Async::NotReady) => match b.poll() {
                    Err(e) => Err(Either::B((e, a))),
                    Ok(Async::Ready(x)) => Ok(Async::Ready(Either::B((x, a)))),
                    Ok(Async::NotReady) => {
                        self.inner = Some((a, b));
                        Ok(Async::NotReady)
                    }
                }
            }
        }
    }

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