error[E0308]: mismatched types
  --> $DIR/invalid_impl_trait_in_assoc_ty.rs:11:28
   |
LL |     type Foo = impl std::fmt::Debug;
   |                -------------------- the expected opaque type
LL |     fn bar() {
LL |         let x: Self::Foo = ();
   |                ---------   ^^ expected opaque type, found `()`
   |                |
   |                expected due to this
   |
   = note: expected opaque type `<() as Foo>::Foo`
                found unit type `()`
note: this item must have the opaque type in its signature in order to be able to register hidden types
  --> $DIR/invalid_impl_trait_in_assoc_ty.rs:10:8
   |
LL |     fn bar() {
   |        ^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.