Type alias used in async fn return type is wrongly reported unused

[Globidev]

With rustc 1.35.0-nightly (26b4cb484 2019-03-09)
The following code

type Bar = ();
async fn foo() -> Bar { }

produces the following warning

warning: type alias is never used: `Bar`                                  
 --> src/main.rs:3:1
  |
3 | type Bar = ();
  | ^^^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default

When Bar is clearly used here.
This seem to only trigger if the alias is used in the return type position

[sanxiyn]

This is unrelated to async fn as one can reproduce with just impl Trait:

trait Trait {
    type Type;
}

impl Trait for () {
    type Type = ();
}

type Bar = ();
fn foo() -> impl Trait<Type = Bar> {}

fn main() {
    foo();
}