Program with HRTB rejected with inexplicable error message

[ytausky]

(Sorry about the non-informative title, I couldn't come up with a useful description.)

As discovered towards the end of this forum thread, the following code (playground) fails to compile:

trait Trait
where
    for<'a> &'a Self::IntoIter: IntoIterator<Item = u32>,
{
    type IntoIter;
    fn get(&self) -> Self::IntoIter;
}

struct Impl(Vec<u32>);

impl Trait for Impl {
    type IntoIter = ImplIntoIter;
    fn get(&self) -> Self::IntoIter {
        ImplIntoIter(self.0.clone())
    }
}

struct ImplIntoIter(Vec<u32>);

impl<'a> IntoIterator for &'a ImplIntoIter {
    type Item = <Self::IntoIter as Iterator>::Item;
    type IntoIter = std::iter::Cloned<std::slice::Iter<'a, u32>>;
    fn into_iter(self) -> Self::IntoIter {
        (&self.0).into_iter().cloned()
    }
}

I get the following error message:

error[E0271]: type mismatch resolving `for<'a> <std::slice::Iter<'a, u32> as std::iter::Iterator>::Item == &u32`
  --> src/lib.rs:13:6
   |
13 | impl Trait for Impl {
   |      ^^^^^ expected bound lifetime parameter 'a, found concrete lifetime
   |
   = note: required because of the requirements on the impl of `std::iter::Iterator` for `std::iter::Cloned<std::slice::Iter<'_, u32>>`

Changing the definition of Item to u32 makes the program compile, but it seems like it should be accepted as it is.

[jakubadamw]

This program is no longer rejected, neither in 1.36, nor in nightly-2019-08-07.

[jakubadamw]

@rustbot modify labels: E-needstest