error[E0700]: hidden type for `Step2<'a>` captures lifetime that does not appear in bounds
  --> $DIR/tait-hidden-erased-unsoundness-2.rs:27:5
   |
LL | pub type Step2<'a> = impl Send + 'a;
   |                      -------------- opaque type defined here
...
LL | pub fn step2<'a, 'b: 'a>() -> Step2<'a>
   |                  -- hidden type `Step1<'a, 'b>` captures the lifetime `'b` as defined here
...
LL |     step1::<'a, 'b>()
   |     ^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0700`.