unused annotation does not silence warning in given using with shorthand

[eejbyfeldt]

Compiler version

3.7.0-RC3

Minimized code

import scala.util.NotGiven
import scala.annotation.unused

object Test {
  given [T](using @unused ev: NotGiven[T <:< Int]): AnyRef with {}
}

Output

Compiled using -Wunused:all

-- [E198] Unused Symbol Warning: unused_notgiven_annotation.scala:5:26 -----------------------
5 |  given [T](using @unused ev: NotGiven[T <:< Int]): AnyRef with {}
  |                          ^^
  |                          unused implicit parameter
1 warning found

Expectation

@unused should silence the warning as it does in other cases.

For example changing the code to

import scala.util.NotGiven
import scala.annotation.unused

object Test {
  given [T](using @unused ev: NotGiven[T <:< Int]): AnyRef = new AnyRef {}
}

works as expected.

[eejbyfeldt]

If #22969 get fixed the test case might need to be changed.

[som-snytt]

Perhaps the annotation winds up on the wrong element in the desugaring.

    given class given_AnyRef[T >: Nothing <: Any](using
      ev: scala.util.NotGiven[<:<[T, Int]]) extends AnyRef() {
      T
      @unused protected given val ev: scala.util.NotGiven[T <:< Int]
    }
    final given def given_AnyRef[T >: Nothing <: Any](using
      @unused ev: scala.util.NotGiven[<:<[T, Int]]): Test.given_AnyRef[T] =
      new Test.given_AnyRef[T](using ev)()

or it is dropped in posttyper.

It's time to revisit this condition:
https://github.com/scala/scala3/blame/main/compiler/src/dotty/tools/dotc/transform/PostTyper.scala#L249

[Gedochao]

Last good release: 3.7.0-RC1-bin-20250127-89c20f8-NIGHTLY
First bad release: 3.7.0-RC1-bin-20250128-9cb97ec-NIGHTLY
c65d45d is the first bad commit (#20894)