rustc reports misc message with impl conflict of trait with AFIT and assoc type.

[Berrysoft]

Code

trait Foo {
    type T;
    
    async fn foo(&self) -> Self::T;
}

struct Bar;

impl Foo for Bar {
    type T = ();

    async fn foo(&self) {}
}

impl Foo for Bar {
    type T = ();

    async fn foo(&self) {}
}

Current output

error[E0284]: type annotations needed: cannot satisfy `<Bar as Foo>::T == ()`
  --> src/main.rs:12:5
   |
12 |     async fn foo(&self) {}
   |     ^^^^^^^^^^^^^^^^^^^ cannot satisfy `<Bar as Foo>::T == ()`

error[E0284]: type annotations needed: cannot satisfy `<Bar as Foo>::T == ()`
  --> src/main.rs:18:5
   |
18 |     async fn foo(&self) {}
   |     ^^^^^^^^^^^^^^^^^^^ cannot satisfy `<Bar as Foo>::T == ()`

For more information about this error, try `rustc --explain E0284`.

Desired output

error[E0119]: conflicting implementations of trait `Foo` for type `Bar`
  --> src/main.rs:11:1
   |
9  | impl Foo for Bar {
   | ---------------- first implementation here
...
15 | impl Foo for Bar {
   | ^^^^^^^^^^^^^^^^ conflicting implementation for `Bar`

For more information about this error, try `rustc --explain E0119`.

Rationale and extra context

No response

Other cases

No response

Anything else?

No response

[compiler-errors]

(for compiler people:) We could fix this by merging the impl WF and coherence checks into one compiler stage. I'll put up a PR and see what T-types thinks.

[traviscross]

@rustbot labels +AsyncAwait-Triaged

We discussed this in a WG-async meeting and are deferring to the resolution @compiler-errors suggested above. He noted that this is a fix to the diagnostics only. That fact that this is an error is correct.