Fix ambiguity issue with associated_type_bounds

[iluuu1994]

Follow up of #61738.

These two particular cases could not be refactored due to an ambiguity issue:

---

rust/src/libcore/iter/adapters/flatten.rs

Lines 70 to 76 in 2d1a551

	impl<I: DoubleEndedIterator, U, F> DoubleEndedIterator for FlatMap<I, U, F>
	where F: FnMut(I::Item) -> U,
	U: IntoIterator,
	U::IntoIter: DoubleEndedIterator
	{
	#[inline]
	fn next_back(&mut self) -> Option<U::Item> { self.inner.next_back() }

With #![feature(associated_type_bounds)]

impl<I: DoubleEndedIterator, U, F> DoubleEndedIterator for FlatMap<I, U, F>
where
    F: FnMut(I::Item) -> U,
    U: IntoIterator<IntoIter: DoubleEndedIterator>,
{
    #[inline]
    fn next_back(&mut self) -> Option<U::Item> { self.inner.next_back() }

Result (line numbers are off):

error[E0221]: ambiguous associated type `Item` in bounds of `U`
   --> src/libcore/iter/adapters/flatten.rs:77:39
    |
77  |     fn next_back(&mut self) -> Option<U::Item> { self.inner.next_back() }
    |                                       ^^^^^^^ ambiguous associated type `Item`
    |
   ::: src/libcore/iter/traits/iterator.rs:94:5
    |
94  |     type Item;
    |     ---------- ambiguous `Item` from `iter::traits::iterator::Iterator`
    |
   ::: src/libcore/iter/traits/collect.rs:211:5
    |
211 |     type Item;
    |     ---------- ambiguous `Item` from `iter::traits::collect::IntoIterator`

---

rust/src/libcore/iter/adapters/flatten.rs

Lines 107 to 110 in 2d1a551

	pub struct Flatten<I: Iterator>
	where I::Item: IntoIterator {
	inner: FlattenCompat<I, <I::Item as IntoIterator>::IntoIter>,
	}

With #![feature(associated_type_bounds)]

pub struct Flatten<I: Iterator<Item: IntoIterator>> {
    inner: FlattenCompat<I, <I::Item as IntoIterator>::IntoIter>,
}

Result (line numbers are off):

error[E0221]: ambiguous associated type `Item` in bounds of `I`
   --> src/libcore/iter/adapters/flatten.rs:112:30
    |
112 |     inner: FlattenCompat<I, <I::Item as IntoIterator>::IntoIter>,
    |                              ^^^^^^^ ambiguous associated type `Item`
    |
   ::: src/libcore/iter/traits/collect.rs:211:5
    |
211 |     type Item;
    |     ---------- ambiguous `Item` from `iter::traits::collect::IntoIterator`
    |
   ::: src/libcore/iter/traits/iterator.rs:94:5
    |
94  |     type Item;
    |     ---------- ambiguous `Item` from `iter::traits::iterator::Iterator`

---

cc @Centril @alexreg

[Centril]

cc also @nikomatsakis

[alexreg]

Thanks for this report. If you/Centril/someone could produce a MCVE, I would then be happy to investigate further... right now I don't have the time to reduce it though, I'm afraid.

[iluuu1994]

I tried again for an hour or two without success. Unfortunately I don't know enough about the internals to understand how this could happen. I'll look at the associated type bounds PR, maybe that will help.