error[E0277]: the trait bound `NotFoo: !Foo` is not satisfied
  --> $DIR/on-unimplemented.rs:9:15
   |
LL | fn hello() -> impl !Foo {
   |               ^^^^^^^^^ the trait bound `NotFoo: !Foo` is not satisfied
LL |
LL |     NotFoo
   |     ------ return type was inferred to be `NotFoo` here

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.