"try expression alternatives have incompatible types" is confusing wording

[comex]

Simple example:

fn foo(x: Result<i32, ()>) -> Result<(), ()> {
    let y: u32 = x?;
    Ok(())
}

Compilation result (rust version 1.44.0-nightly (52fa23add 2020-04-18)):

error[E0308]: try expression alternatives have incompatible types
 --> src/lib.rs:2:18
  |
2 |     let y: u32 = x?;
  |                  ^^ expected `u32`, found `i32`
  |
help: you can convert an `i32` to `u32` and panic if the converted value wouldn't fit
  |
2 |     let y: u32 = x?.try_into().unwrap();
  |                  ^^^^^^^^^^^^^^^^^^^^^^

Parts of this diagnostic are great, showing the mismatched types (u32 and i32) and even suggesting a valid possible fix. But the wording of the error itself is confusing. What "try expression alternatives"? I don't see any alternatives.

Taking a glance at the compiler implementation, it's referring to the arms of the match block that ? desugars into. But it's confusing to talk about those, because:

• They're not visible to the user.

• Why would there be a type mismatch between the arms in the first place, if there are only two arms and one of them always ends in a return rather than producing a value? Did type inference infer the error branch as having type u32 due to the context?

  If so, the message is technically accurate, but unhelpful: from the user's point of view, the 'real' issue is a mismatch between the expected type of x? (namely i32) and the context. As far as I can imagine, this is the only possible cause of a type mismatch between the arms of a desugared ?. So it seems like it would be fine to just change the error message to not mention the arms.

As a sidenote, if I take the -Zunpretty=hir output and try to compile that:

#![feature(try_trait)]
use std::ops::Try;
use std::convert::From;
fn foo(x: Result<i32, ()>) -> Result<(), ()> {
    let y: u32 = match Try::into_result(x) {
        Ok(val) => val,
        Err(err) => return Try::from_error(From::from(err)),
    };
    Ok(())
}

...oddly enough, I don't get a "match arms have incompatible types" error, but a more helpful one instead:

error[E0308]: mismatched types
 --> q.rs:6:20
  |
6 |         Ok(val) => val,
  |                    ^^^ expected `u32`, found `i32`
  |
help: you can convert an `i32` to `u32` and panic if the converted value wouldn't fit
  |
6 |         Ok(val) => val.try_into().unwrap(),
  |                    ^^^^

[estebank]

This is because of the explicit check here

rust/src/librustc_infer/infer/error_reporting/mod.rs

Lines 2079 to 2089 in d7c4081

	MatchExpressionArm(box MatchExpressionArmCause { source, .. }) => {
	Error0308(match source {
	hir::MatchSource::IfLetDesugar { .. } => {
	"`if let` arms have incompatible types"
	}
	hir::MatchSource::TryDesugar => {
	"try expression alternatives have incompatible types"
	}
	_ => "`match` arms have incompatible types",
	})
	}