-Wunused:all and higher kinded type type parameter

[amumurst]

I tried adding the new unused flag (from #16157) to my work codebase and was surprised that it warned about unused import when used with the cats libraries IO type. A minimal reproducer independent of cats is below.

Compiler version

With nightly: 3.3.0-RC1-bin-20230112-be10bc6-NIGHTLY

Minimized code

//> using scala "3.3.0-RC1-bin-20230112-be10bc6-NIGHTLY"
//> using option "-Wunused:all"

object myPackage:
   case class HKT[A](a: A)

trait Thing[F[_]]
import myPackage.HKT
val x = new Thing[HKT]{}

Output

[warn] ./unused-higher-order-type-param.sc:8:18: unused import
[warn] import myPackage.HKT
[warn]                  ^^^

Expectation

HKT is used as the instance of F in Thing, and hence should not be reported as unused.

[szymon-rd]

Thank you for reporting all these errors! We will promptly address them :)

[amumurst]

No worries, I am working my way through the app hacking around them for now, happy to help out 👍

[cb372]

I beat you to this one by a few minutes 😄

This is the same issue as #16680

[dwijnand]

Duplicate of #16680