error[E0700]: hidden type for `Opaque<'a, T>` captures lifetime that does not appear in bounds
  --> $DIR/missing_lifetime_bound.rs:4:47
   |
LL | fn defining<'a, T>(x: &'a i32) -> Opaque<T> { x }
   |             --                                ^
   |             |
   |             hidden type `&'a i32` captures the lifetime `'a` as defined here

error: aborting due to previous error

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