error[E0271]: type mismatch resolving `<impl Future<Output = [async output]> as Future>::Output == impl Stream<Item = Repr>`
  --> $DIR/issue-89008.rs:39:43
   |
LL |     type LineStream<'a, Repr> = impl Stream<Item = Repr>;
   |                                 ------------------------ the expected opaque type
...
LL |     fn line_stream<'a, Repr>(&'a self) -> Self::LineStreamFut<'a, Repr> {
   |                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected opaque type, found struct `Empty`
   |
   = note: expected opaque type `impl Stream<Item = Repr>`
                   found struct `Empty<_>`

error: could not find defining uses
  --> $DIR/issue-89008.rs:35:33
   |
LL |     type LineStream<'a, Repr> = impl Stream<Item = Repr>;
   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0271`.