failing orphan check with associated types should suggest using the associated type directly.

[lcnr]

trait Trait {
    type Assoc;
}

struct LocalTy;
impl Trait for () {
    type Assoc = LocalTy;
}

impl PartialEq for <() as Trait>::Assoc {
    fn eq(&self, other: &Self) -> bool {
        true
    }
}

results in

error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
  --> src/lib.rs:10:1
   |
10 | impl PartialEq for <() as Trait>::Assoc {
   | ^^^^^---------^^^^^--------------------
   | |    |             |
   | |    |             `<() as Trait>::Assoc` is not defined in the current crate
   | |    `<() as Trait>::Assoc` is not defined in the current crate
   | impl doesn't use only types from inside the current crate
   |
   = note: define and implement a trait or new type instead

ideally it should mention that <() as Trait>::Assoc may actually be local to the current crate and that the user could replace that projection with the associated type itself.

[in42]

@rustbot claim

[fmease]

The snippet compiles since #117164 (1.80) as <() as Trait>::Assoc now gets normalized. Closing as not planned as a diagnostic is no longer necessary.