Description
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.