false-positive "unused import" warning

[xuwei-k]

Compiler version

• 3.6.0-RC1-bin-20240828-5e83606-NIGHTLY
• 3.5.1-RC2
• 3.5.0
• 3.4.3
• 3.3.3

Minimized example

build.sbt

scalaVersion := "3.5.1-RC2"

scalacOptions += "-Wunused:imports"

A.scala

import scala.reflect.TypeTest

trait A {
  type B
  type C <: B

  given instance: TypeTest[B, C]
}

def f(a: A, b: a.B): Boolean = {
  import a.C
  b match {
    case _: C =>
      true
    case _ =>
      false
  }
}

Output Error/Warning message

[warn] -- [E198] Unused Symbol Warning: false-unused-import-warn-example-project/A.scala:11:11 
[warn] 11 |  import a.C
[warn]    |           ^
[warn]    |           unused import

Why this Error/Warning was not helpful

a.C is not unused

Suggested improvement

don't warn

[mbovel]

This issue was picked for the Scala Issue Spree of tomorrow, June 24th. @KuceraMartin and @iusildra will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.

Note: it's hard for me to estimate the difficulty of this issue, as I am quite unfamiliar with the unused imports warning.

[KuceraMartin]

It turned out this is not trivial to fix. During the typer phase, the information about C being used is lost:

package <empty> {
  import scala.reflect.TypeTest
  trait A() extends Object {
    type B >: Nothing <: Any
    type C >: Nothing <: Any
    given def instance: scala.reflect.TypeTest[A.this.B, A.this.C]
  }
  final lazy module val 21525$package: 21525$package = new 21525$package()
  final module class 21525$package() extends Object() {
    this: 21525$package.type =>
    def f(a: A, b: a.B): Boolean =
      {
        import a.C
        b match
          {
            case a.instance(_) =>
              true
            case _ =>
              false
          }
      }
  }
}

Since the CheckUnused phase happens after typer, it doesn't see any usage of C and therefore it emits a warning.

To fix this, we would probably need to store some additional information when erasing C in the typer phase.