Spurious type deduced when using `?`

[yshui]

I tried this code:

#![feature(associated_type_bounds)]
struct S;
fn fn1<I: IntoIterator<Item: AsRef<S>>>(_iter: I) -> Result<(), ()> {
    Ok(())
}

async fn fn2(s: &str) -> Result<(), ()> {
    let s = s.parse().map_err(|_| ())?; // <- Spurious type deduced
    fn1(std::iter::once(s)).map_err(|e| ())?;
    Ok(())
}

I expected to see this happen:

error[E0282]: type annotations needed, because type of s cannot be determined.

Instead, this happened:

error[E0277]: the trait bound '(): std::str::FromStr' is not satisfied

As if s is assigned type ()

[yshui]

Looks like ? triggers the problem. Also this has nothing to do with associated type.

This has the problem:

fn fn2(s: &str) -> Result<(), ()> {
    let _ = s.parse().map_err(|_| ())?;
    Ok(())
}

But not this:

fn fn2(s: &str) {
    let _ = s.parse().unwrap();
}

[yshui]

I think label "F-associated_type_bounds", "requires-nightly" should be removed. "A-diagnostics" should be added.

[jackh726]

Minimal repro

fn fn2(s: &str) -> Result<(), ()> {
    let s = s.parse().map_err(|_| ())?;
    Ok(())
}

with

error[E0277]: the trait bound `(): FromStr` is not satisfied
 --> src/lib.rs:2:15
  |
2 |     let s = s.parse().map_err(|_| ())?;
  |               ^^^^^ the trait `FromStr` is not implemented for `()`

I agree with @yshui here: this is really just a diagnostics issue. s here is falling back to () since it's isn't used after being reassigned.