Propagate never type

[adeschamps]

The following fails to compile:

use failure::Error;

fn main() -> Result<(), Error> {
    let success: Result<(), !> = Ok(());
    success?;
    Ok(())
}

error[E0277]: the trait bound `(): std::error::Error` is not satisfied
 --> src/main.rs:8:12
  |
8 |     success?;
  |            ^ the trait `std::error::Error` is not implemented for `()`
  |
  = note: required because of the requirements on the impl of `failure::Fail` for `()`
  = note: required because of the requirements on the impl of `std::convert::From<()>` for `failure::error::Error`
  = note: required by `std::convert::From::from`

It can be made to compile by mapping the error type:

success.map_err(|e| format_err!("never: {}", e))?;

... but that results in a warning for unreachable code, since the error type is !.

Is there a workaround for this? Right now I'm calling unwrap(), and I put some explicit type annotations so if the error type ever changes, It'll fail to compile.