-Wunused against lila

[som-snytt]

Compiler version

3.7.0-RC1-bin-SNAPSHOT

Minimized code

object Bus:

  type Payload            = Matchable
  type SubscriberFunction = PartialFunction[Payload, Unit]

  def pub[T <: Payload](payload: T)(using wc: WithChannel[T]) =
    publish(payload, wc.channel)

  def sub[T <: Payload: Typeable](f: PartialFunction[T, Unit])(using wc: WithChannel[T]) =
    subscribeFun(wc.channel):
      case x: T => f.applyOrElse(x, _ => ())

Output (click arrow to expand)

[error] java.lang.IndexOutOfBoundsException: 1
[error]         at scala.collection.LinearSeqOps.apply(LinearSeq.scala:131)
[error]         at scala.collection.LinearSeqOps.apply$(LinearSeq.scala:128)
[error]         at scala.collection.immutable.List.apply(List.scala:79)
[error]         at dotty.tools.dotc.transform.CheckUnused.transformSelect(CheckUnused.scala:72)
[error]         at dotty.tools.dotc.transform.CheckUnused.transformSelect(CheckUnused.scala:67)
[error]         at dotty.tools.dotc.transform.MegaPhase.goSelect(MegaPhase.scala:636)

which is

  override def transformSelect(tree: Select)(using Context): tree.type =
    val name = tree.removeAttachment(OriginalName).getOrElse(nme.NO_NAME)
    if tree.span.isSynthetic && tree.symbol == defn.TypeTest_unapply then
      tree.qualifier.tpe.underlying.finalResultType match
      case AppliedType(_, args) => // tycon.typeSymbol == defn.TypeTestClass
        val res = args(1) // T in TypeTest[-S, T]

Maybe because of the alias. The code here assumes there are two type args.

[som-snytt]

This was to test the rewrite.

ThisBuild / scalacOptions ++= Seq("-unchecked", "-deprecation", "-Wunused:all", "-rewrite")

and

val globalScalaVersion = "3.7.0-RC1-bin-SNAPSHOT"

at c95f43e1f50f2d247c4d9a129faa55d46d19159d