Associated type equality not followed through multiple traits

[sgrif]

Simplest case to reproduce would be to add a trait which decorates IntoIterator and try to write a blanket impl for that trait:

trait DecoratesIntoIterator: IntoIterator {
    type Output: Iterator<Item=Self::Item>;
}

impl<T> DecoratesIntoIterator for T where
    T: IntoIterator,
    T::IntoIter: DecoratesIntoIterator,
{
    type Output = <T::IntoIter as DecoratesIntoIterator>::Output;
}

Will fail to compile with

type mismatch resolving `<<<T as std::iter::IntoIterator>::IntoIter as DecoratesIntoIterator>::Output as std::iter::Iterator>::Item == <T as std::iter::IntoIterator>::Item`:

even though every trait mentioned has the constraint that the output has Iterator<Item=Self::Item>.

Ran into this with a more concrete use case inside of Diesel:

trait Query {
    type SqlType;
}

trait AsQuery {
    type SqlType;
    type Query: Query<SqlType=Self::SqlType>;

    fn as_query(self) -> Self::Query;
}

trait LimitDsl: AsQuery {
    type Output: Query<SqlType=Self::SqlType>;

    fn limit(self, limit: i64) -> Self::Output;
}

impl<T> LimitDsl for T where
    T: AsQuery,
    T::Query: LimitDsl,
{
    type Output = <T::Query as LimitDsl>::Output;

    fn limit(self, limit: i64) -> Self::Output {
        self.as_query().limit(limit)
    }
}

This can be worked around by manually restating the constraint, like so:

impl<T, ST> LimitDsl for T where
    T: AsQuery<SqlType=ST>,
    T::Query: LimitDsl<SqlType=ST>,

but this shouldn't be required.

[TimNN]

I believe that this may have the same cause as #25409 (the "This can be worked around by manually restating the constraint" sounds familiar at least), event though the issue manifests itself in a different way here.

[steveklabnik]

Triage; still reproduces