Regression in type resolution in #[tokio::main] macro

[KamilaBorowska]

Version
List the versions of all tokio crates you are using. The easiest way to get
this information is using cargo tree subcommand:

cargo tree | grep tokio

└── tokio v1.12.0
    └── tokio-macros v1.5.0 (proc-macro)

Platform
The output of uname -a (UNIX), or version and 32 or 64-bit (Windows)

Linux nixos 5.10.71 #1-NixOS SMP Wed Oct 6 13:56:04 UTC 2021 x86_64 GNU/Linux

Description
Enter your issue details here.
One way to structure the description:

Ending async fn main() -> ! (where ! is any termination type other than ()) function with panic!(); causes compilation failure.

I tried this code:

#[tokio::main]
async fn main() -> ! {
    panic!();
}

I expected to see this happen: Compilation to succeed

Instead, this happened:

error[E0308]: mismatched types
 --> src/main.rs:2:20
  |
1 | #[tokio::main]
  |              - help: consider removing this semicolon
2 | async fn main() -> ! {
  |          ----      ^ expected `!`, found `()`
  |          |
  |          implicitly returns `()` as its body has no tail or `return` expression
  |
  = note:   expected type `!`
          found unit type `()`

This is a regression in tokio-macros 1.4.1, this code did work fine with tokio-macros 1.3.0. It affected tests in my code, see https://gitlab.com/KonradBorowski/showdown/-/blob/413fac7fdcd1b22ad9a6fbf495a79ae5d8e1473c/src/lib.rs#L187 for an example of affected test.

[taiki-e]

This will be fixed in #4176.