error[E0308]: mismatched types
  --> $DIR/higher-ranked-upcasting-ub.rs:22:5
   |
LL | fn unsound(x: &dyn for<'a> Subtrait<'a, 'a>) -> &dyn for<'a, 'b> Supertrait<'a, 'b> {
   |                                                 ----------------------------------- expected `&dyn for<'a, 'b> Supertrait<'a, 'b>` because of return type
LL |     x
   |     ^ expected trait `Supertrait`, found trait `Subtrait`
   |
   = note: expected reference `&dyn for<'a, 'b> Supertrait<'a, 'b>`
              found reference `&dyn for<'a> Subtrait<'a, 'a>`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0308`.