HKL issues with deep associated types.

[pierzchalski]

Using HKL and associated types, one can write constraints that should be accepted which the compiler rejects (mostly using the IntoIterator: IntoIter: Iterator: IteratorWithSomeProperty hierarchy).

'Motivating' example here and a minimal one here. The intermediate trait is required to show the error.

To reiterate what's in the comments of the first example, consider the following trait and function:

trait Tr {}
impl<A> Tr for Vec<A> {}

fn fancy_constraints<A, T: Tr>(t: T)
  where for<'a> &'a T: IntoIterator,
        for<'a> <&'a T as IntoIterator>::IntoIter: ExactSizeIterator,
{} 

Does Vec<A> satisfy these constraints? &'a Vec<A> has an IntoIterator implementation via Deref<Target=[A]> (this indirection isn't the issue: we can make a newtype wrapper around Vec<A> which directly implements IntoIterator and still exhibits the problem).

We have <&'a Vec<A> as IntoIterator>::IntoIter == Iter<'a, A>, and Iter<'a, A>: ExactSizeIterator>, so this should satisfy the constraints given in fancy_constraints above. Instead, we get the following error message:

error: the trait bound `for<'a> <&'a _ as std::iter::IntoIterator>::IntoIter: std::iter::ExactSizeIterator` is not satisfied [--explain E0277]
  --> <anon>:13:5
13 |>     fancy_constraints(vec![0u8]);
   |>     ^^^^^^^^^^^^^^^^^
help: the following implementations were found:
help:   <&'a mut I as std::iter::ExactSizeIterator>
note: required by `fancy_constraints`

The help message is somewhat confusing (I couldn't find the implementation it mentions, but I didn't look very hard), and I couldn't find a way to use it to fix the problem.

[Mark-Simulacrum]

The example given compiles with only unused code warnings today, so closing.