Defaulted method with `self: Arc<Self>` doesn't work

[Chayim Refael Friedman]

Arc generates a where Self: Send bound, but for Arc it is not enough - it requires also T: Sync to be Send. This make the following code failing to compile:

#[async_trait]
trait Trait {
    async fn foo(self: Arc<Self>) {}
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=add1c3d15459405538a453e3c9a504a6

error: future cannot be sent between threads safely
 --> src/lib.rs:7:35
  |
7 |     async fn foo(self: Arc<Self>) {}
  |                                   ^^ future created by async block is not `Send`
  |
note: captured value is not `Send`
 --> src/lib.rs:7:18
  |
7 |     async fn foo(self: Arc<Self>) {}
  |                  ^^^^ has type `Arc<Self>` which is not `Send`
  = note: required for the cast to the object type `dyn Future<Output = ()> + Send`

Adding Sync as a supertrait works, but ideally the macro should also add a Self: Sync bound (even though I don't know how it can identify Arc being used), or at least document that.