error[E0720]: cannot resolve opaque type
  --> $DIR/where-allowed-2.rs:6:30
   |
LL | fn in_adt_in_return() -> Vec<impl Debug> { panic!() }
   |                              ^^^^^^^^^^    -------- this returned value is of `!` type
   |                              |
   |                              cannot resolve opaque type
   |
   = help: this error will resolve once the item's body returns a concrete type

error: aborting due to previous error

For more information about this error, try `rustc --explain E0720`.