error: non-defining opaque type use in defining scope
  --> $DIR/issue-68368-non-defining-use.rs:11:15
   |
LL | fn f<'a>() -> Alias<'a, ()> {}
   |               ^^^^^^^^^^^^^
   |
note: used non-generic type `()` for generic parameter
  --> $DIR/issue-68368-non-defining-use.rs:10:16
   |
LL | type Alias<'a, U> = impl Trait<U>;
   |                ^

error: aborting due to previous error