Different error messages between async and regular functions

[Patryk27]

Hi,

Let's consider following code:

fn foo() -> Result<bool, ()> {
    unimplemented!()
}

fn bar() -> Result<(), ()> {
    foo()?;
    ()
}

There's an error in the penultimate line - it should say Ok(()) and Rust correctly helps to spot the mistake:

  |
7 | fn bar() -> Result<(), ()> {
  |             -------------- expected `std::result::Result<(), ()>` because of return type
8 |     foo()?;
9 |     ()
  |     ^^ expected enum `std::result::Result`, found ()
  |
  = note: expected type `std::result::Result<(), ()>`
             found type `()`

The situation changes a bit when we switch to async functions:

#![feature(async_await)]

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

async fn bar() -> Result<(), ()> {
    foo().await?;
    ()
}

The error message now says the '?' operator can only be used in a function that returns 'Result' or 'Option', which is totally unhelpful, since the function is (or at least should be) returning a Result, according to its signature.

It seems that, when dispatching error messages, the actual return types have precedence over signatures in async functions, but it's exactly the other way around in regular functions, which leads to kinda unexpected and confusing error messages.

[Patryk27]

For reference, it seems it's a duplicate of: #54326.

[cramertj]

Closing as duplicate.