Async function return type inference fails without an ending return

[khionu]

I have a function of the signature async fn run() -> Result<!, Box<dyn std::error::Error>>. It ends with a loop that, as you may infer, never returns Ok. The inference for code generation failed, and the various compiler errors said that my function actually returned ().

[memoryruins]

Can you provide a function/ playground that reproduces the errors?

[khionu]

In trying to get a solid reproduction, the issue has disappeared from the codebase I discovered it in. 👻

[aleksanb]

I managed to reproduce this on stable (cargo 1.39.0 (1c6ec66d5 2019-09-30)), but it seems to be fixed on my current nightly (cargo 1.40.0-nightly (5da4b4d47 2019-10-28)). The failing code is:

async fn compile_fail_without_return() -> Result<i32, i32> {
    Result::<i32, i32>::Ok(20i32)?;
    //Result::<i32, i32>::Ok(20i32) If you add me, stable gives a sane message.
}

fn main() {}

This is the error on stable:

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
  --> src/main.rs:27:5
   |
27 |     Result::<i32, i32>::Ok(20i32)?;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
   |
   = help: the trait `std::ops::Try` is not implemented for `()`
   = note: required by `std::ops::Try::from_error`

error[E0271]: type mismatch resolving `<impl std::future::Future as std::future::Future>::Output == std::result::Result<i32, i32>`
  --> src/main.rs:26:43
   |
26 | async fn compile_fail_without_return() -> Result<i32, i32> {
   |                                           ^^^^^^^^^^^^^^^^ expected (), found enum `std::result::Result`
   |
   = note: expected type `()`
              found type `std::result::Result<i32, i32>`
   = note: the return type of a function must have a statically known size

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0271, E0277.
For more information about an error, try `rustc --explain E0271`.
error: could not compile `k`.

This is the error on nightly:

  --> src/main.rs:26:60
   |
26 |   async fn compile_fail_without_return() -> Result<i32, i32> {
   |  ____________________________________________________________^
27 | |     Result::<i32, i32>::Ok(20i32)?;
28 | |     //Result::<i32, i32>::Ok(20i32)
29 | | }
   | |_^ expected enum `std::result::Result`, found ()
   |
   = note: expected type `std::result::Result<i32, i32>`
              found type `()`

It seems like the function return type is clobbered somehow in the async transformation.