Error conversion with !

[y86-dev]

I tried this code:

#![feature(never_type)]

fn no_fail() -> Result<(), !> {
    Ok(())
}

fn main() -> Result<(), ()> {
    no_fail()?;
    Ok(())
}

I expected to see this happen: successful compilation because of the blanket impl<T> From<!> for T

Instead, this happened:

error[E0277]: `?` couldn't convert the error to `()`
 --> src/main.rs:8:14
  |
7 | fn main() -> Result<(), ()> {
  |              -------------- expected `()` because of this
8 |     no_fail()?;
  |              ^ the trait `From<!>` is not implemented for `()`
  |
  = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
  = help: the following other types implement trait `FromResidual<R>`:
            <Result<T, F> as FromResidual<Result<Infallible, E>>>
            <Result<T, F> as FromResidual<Yeet<E>>>
  = note: required for `Result<(), ()>` to implement `FromResidual<Result<Infallible, !>>`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `tmp` due to previous error

Meta

rustc --version --verbose:

rustc 1.65.0-nightly (1120c5e01 2022-09-08)
binary: rustc
commit-hash: 1120c5e01df508de64fe6642f22fadeb574afd6d
commit-date: 2022-09-08
host: x86_64-unknown-linux-gnu
release: 1.65.0-nightly
LLVM version: 15.0.0

I cannot really make sense of the error message, as it complains about a missing From impl. Maybe this is an issue with the Try implementation (try_trait_v2 does not change the error) but I cannot tell.

[Jules-Bertholet]

blanket impl<T> From<!> for T

This doesn't exist. It would conflict with impl<T> From<T> for T

[y86-dev]

I thought this was the impl: https://doc.rust-lang.org/src/core/convert/mod.rs.html#574-578

However I did not see the #[rustc_reservation_impl] (this was not visible in the rustdoc, leading to my confusion). #64715 is the related issue, if anyone is interested.

Going to close this as this should be tracked by the other issue.