False positive unused warning with `-Wunused:params` and `extension`

[mrdziuban]

Compiler version

3.3.0-RC3

Minimized code

https://scastie.scala-lang.org/mrdziuban/nBgViUmQQkiecLQSHAe71w/12

type Test[A] = A

extension [A](x: Test[A]) {
  def value: A = x
  def causesIssue: Unit = println("oh no")
}

Output

extension [A](x: Test[A]) {
           // ^
           // unused explicit parameter

Expectation

Since x is used in def value the parameter should not be reported as unused. This only happens when def causesIssue is present, so I'm guessing that this is related to how extension methods are desugared and the unused warning is because that particular method doesn't use x.

[SethTisue]

reported again today on Discord

[szymon-rd]

For now, I made a PR that skips the extension methods altogether: #17178