error[E0069]: `return;` in a function whose return type is not `()`
  --> $DIR/return-unit-from-diverging.rs:5:5
   |
LL | fn fail() -> ! {
   |              - expected `!` because of this return type
LL |     return;
   |     ^^^^^^ return type is not `()`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0069`.