Using a type alias in an impl block header doesn't mark it as being used

[glandium]

Reduced testcase courtesy of Arnavion on irc:

pub struct Foo;

type Bar = Foo;

impl Bar {
}

The compiler prints the following error:

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

[IslandUsurper]

Duplicate of #47131, perhaps.

[glandium]

Indeed, duplicate of #47131.