error: non-defining opaque type use in defining scope
  --> $DIR/missing_lifetime_bound.rs:4:47
   |
LL | fn defining<'a, T>(x: &'a i32) -> Opaque<T> { x }
   |                                               ^ lifetime `'a` is part of concrete type but not used in parameter list of the `impl Trait` type alias

error: aborting due to previous error