error: type parameter `impl Deref<Target = Self>` is part of concrete type but not used in parameter list for the `impl Trait` type alias
  --> $DIR/issue-57700.rs:19:58
   |
LL |       fn foo(self: impl Deref<Target = Self>) -> Self::Bar {
   |  __________________________________________________________^
LL | |
LL | |         self
LL | |     }
   | |_____^

error: aborting due to previous error