-Wunused: false positive for pure expressions in for-yield

[hejfelix]

Compiler version

3.3.0-RC2

Minimized code

  for
    x <- List(1,2,3) // unused pattern variable warning
    y = x * 2
  yield y

Expectation

no warning since the pattern variable is used in the pure expression

[SethTisue]

same as scala/bug#10287 perhaps? cc @som-snytt

[som-snytt]

The comment on the feature PR (at some point) was that fidelity of representation should make analysis easier in Scala 3, so hopefully this ticket is not as discouraging.

[hejfelix]

same as scala/bug#10287 perhaps? cc @som-snytt

Doesn't seem to be exactly the same, but could be same root cause?

[SethTisue]

What compiler settings are needed to reproduce this? I'm not able to reproduce it with:

//> using scala "3.3.0-RC2"
//> using option "-Wunused:all"

object O:
  for
    x <- List(1,2,3)
    y = x * 2
  yield y

[som-snytt]

@SethTisue not sure how output works, but

➜   ./bin/scala -Wunused:privates
Welcome to Scala 3.3.1-RC1-bin-SNAPSHOT-git-8c616bf (19, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> class C { private def f = 42 }
Formatted members List()
// defined class C

scala>
➜  ./bin/scalac -Wunused:privates -d /tmp c.scala
-- Warning: c.scala:2:22 -----------------------------------------------------------------------------------------------
2 |class C { private def f = 42 }
  |                      ^
  |                      unused private member
1 warning found

[SethTisue]

same result (no warning) with -Wunused:privates (which one would hope -Wunused:all would include, anyway)

[som-snytt]

I intended to show that if REPL reporter doesn't report, it's conceivable that the command runner reporter doesn't report, so possibly only scalac reports? and maybe metals?

[hejfelix]

I tried to minimize it, but I can't seem to provoke it now. I'll see if it persists in my larger project.

[hejfelix]

It seems to happen in cross-projects:

// plugins.sbt
addSbtPlugin("org.scala-js"              % "sbt-scalajs"              % "1.13.0")
addSbtPlugin("org.portable-scala"        % "sbt-scalajs-crossproject" % "1.2.0")


// build.sbt
val scala3Version = "3.3.0-RC2"

lazy val root = crossProject(JSPlatform,JVMPlatform)
  .in(file("."))
  .settings(
    name := "wunused-repro",
    version := "0.1.0-SNAPSHOT",
    scalaVersion := scala3Version,
    scalacOptions ++= Seq("-Wunused:unsafe-warn-patvars")
  )

// jvm/src/main/scala/Main.scala

@main def run = println("HEY")

val xs =
  for 
    x <- List(1,2,3) // unused pattern variable
    y = x * 2
  yield y