type alias incorrectly flagged as unused when solely used in impl header

[pnkfelix]

Consider the following code (play):

type T = ();

struct S<X>(X); 

impl Clone for S<T> { fn clone(&self) -> Self { S(()) } }

fn main() {
    let s = S(());
    drop(s.clone());
}

Today this emits the following warning diagnostic:

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

But that type alias is not dead code. It is used in the impl Clone for S<T> { ... }, as one can see by trying to recompile the code after commenting out the type alias.

[oli-obk]

Previous instances of related (or possibly the same) problems:

• Unused warning for type alias doesn't take into account inherent impls. #47131
• Incorrect warning: type alias is never used #54234

[varkor]

I think this is essentially the same problem as #59333 (comment).

[istankovic]

Seems to be fixed. Tested with 1.73 and got no warnings.