error: defining opaque type use does not fully define opaque type: generic parameter `U` is specified as concrete type `()`
  --> $DIR/issue-68368-non-defining-use.rs:8:1
   |
LL | fn f<'a>() -> Alias<'a, ()> {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: could not find defining uses
  --> $DIR/issue-68368-non-defining-use.rs:7:1
   |
LL | type Alias<'a, U> = impl Trait<U>;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors