Missing unused warnings for lambdas parameters

[mbovel]

In Scala 2.13:

object Foo {
  val dummy = 42
  def f(): Unit = Option(1).map((x: Int) => dummy)
  def main(args: Array[String]): Unit = {}
}

➜  ~/scala-snippets-6 scala-cli -Ywarn-unused -S 2.13 warning_lamdas.scala
[warn] ./warning_lamdas.scala:3:34
[warn] parameter x in anonymous function is never used
[warn]   def f(): Unit = Option(1).map((x: Int) => dummy)

In Scala 3, no warning.

Might be due to #17223.

[jan-pieter]

@mbovel I couldn't let go and have taken a stab at this in #20973 😉. WDYT?

[Gedochao]

@mbovel what should be the flag to be passed to the Scala 3 compiler, when the warning should be logged?
-Wunused:all and -Wunused:params?

[som-snytt]

I know I encountered this while enabling lints on the scala 2 code base. Sorry I don't have a link, but I'll add it if I find it.

I think my conclusion was, "naming is hard, avoid it if you can."

xs.map((_: Int) => f) // let's say the type is required

Since naming is a cost (like anything else), penalize it!

"If you name it, use it!"

If Scala had a mascot, Scala Center could commission a poster showing the mascot with that slogan. Like Smokey Bear saying only you can prevent forest fires.

[som-snytt]

Lambda parameters will warn even for trivial bodies. x => ???.