error[E0277]: `?` couldn't convert the error to `()`
  --> $DIR/issue-32709.rs:4:11
   |
LL | fn a() -> Result<i32, ()> {
   |           --------------- expected `()` because of this
LL |     Err(5)?;
   |     ------^ the trait `From<{integer}>` is not implemented for `()`
   |     |
   |     this can't be annotated with `?` because it has type `Result<_, {integer}>`
   |
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
   = help: the following other types implement trait `From<T>`:
             <(T,) as From<[T; 1]>>
             <(T, T) as From<[T; 2]>>
             <(T, T, T) as From<[T; 3]>>
             <(T, T, T, T) as From<[T; 4]>>
             <(T, T, T, T, T) as From<[T; 5]>>
             <(T, T, T, T, T, T) as From<[T; 6]>>
             <(T, T, T, T, T, T, T) as From<[T; 7]>>
             <(T, T, T, T, T, T, T, T) as From<[T; 8]>>
           and 4 others
   = note: required for `Result<i32, ()>` to implement `FromResidual<Result<Infallible, {integer}>>`

error: aborting due to 1 previous error

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