Unreachable code generated by try_join! with never error

[Nemo157]

This is currently causing CI failures because one of the tests inadvertently triggers it.

Given a crate with:

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

pub async fn bar() {
    let _: Result<((), ()), !> = futures::try_join!(foo(), foo());
}

compiling it produces two unreachable call warnings pointing at the try_join!:

warning: unreachable call
 --> src/lib.rs:6:34
  |
6 |     let _: Result<((), ()), !> = futures::try_join!(foo(), foo());
  |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |                                  |
  |                                  unreachable call
  |                                  any code following this expression is unreachable
  |
  = note: `#[warn(unreachable_code)]` on by default
  = note: this warning originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

(-Z external-macro-backtrace doesn't give anything useful).

[Nemo157]

(This recently started failing in CI because the FutureExt::never_error combinator started producing a true ! instead of the temporary core::convert::Infallible, and it appears rustc has better dead code detection (or a bug) based on this specific type).