Nonconflicting Conflicting impl in trait object when trait and object impl same item name

[Havvy]

Playpen

Source

trait Trait {
    fn test(&self) -> &'static str { "Default" }
}

struct NotSyncStruct(*const ());

impl NotSyncStruct {
    fn new() -> NotSyncStruct { NotSyncStruct(&() as *const ()) }
}

impl Trait {
    fn test(&self) -> &'static str { "Without Send and Sync" }
}

impl Trait + Send + Sync {
    fn test(&self) -> &'static str { "With Send and Sync" }
}

impl Trait for NotSyncStruct {}

fn main () {
    println!("{}", <Trait>::test(&NotSyncStruct::new()));
}

Error Message

   Compiling playground v0.0.1 (file:///playground)
error[E0034]: multiple applicable items in scope
  --> src/main.rs:22:20
   |
22 |     println!("{}", <Trait>::test(&NotSyncStruct::new()));
   |                    ^^^^^^^^^^^^^ multiple `test` found
   |
note: candidate #1 is defined in an impl for the type `Trait + 'static`
  --> src/main.rs:12:5
   |
12 |     fn test(&self) -> &'static str { "Without Send and Sync" }
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: candidate #2 is defined in an impl for the type `Trait + std::marker::Sync + std::marker::Send + 'static`
  --> src/main.rs:16:5
   |
16 |     fn test(&self) -> &'static str { "With Send and Sync" }
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: candidate #3 is defined in the trait `Trait`
  --> src/main.rs:2:5
   |
2  |     fn test(&self) -> &'static str { "Default" }
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = help: to disambiguate the method call, write `Trait::test(...)` instead

error: aborting due to previous error

error: Could not compile `playground`.

To learn more, run the command again with --verbose.

The error message here suggests that there are three implementations that define the associated item when in reality, there can only be two. It's not possible to call the implementation on Trait + std::marker::Sync + std::marker::Send + 'static` because the type is not Sync. So the error message should not show that implementation.

At least, that is what I believe the error to be. For if you comment out either the default implementation on line 2 or the inherent implementation on line 12, then the program actually compiles.

[Havvy]

Actually, I'm not even sure there should be a conflict here. Inherent methods are supposed to win out over all trait methods. And since the implementations don't seem to overlap, I can only presume that the conflicting implementation are false altogether.

[petrochenkov]

@Havvy

Inherent methods are supposed to win out over all trait methods.

Trait objects are a special case, trait method and inherent method on a trait with the same name are supposed to conflict because someone (IIRC, @nrc) in the past thought it would prevent errors.
There's even test checking for this, but I can't find it without modifying the compiler.
I think this can be changed backward-compatibly though.

[Havvy]

If that's the case, then the error should happen without having to call the function. I'd be okay with making it a hard error to have a trait object implement an item with the same name as its trait. Just make sure there's a specific error for this. Although with dyn Trait becoming a thing, I don't think there would be too much confusion, and could just be relegated to a lint (possibly even a Clippy lint?)

[Havvy]

This still needs to be tagged. cc @kennytm

[Havvy]

Closing in favor of issue #51402