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False positive unused given in extensions #23125

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WojciechMazur opened this issue May 8, 2025 · 4 comments · May be fixed by #23132
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False positive unused given in extensions #23125

WojciechMazur opened this issue May 8, 2025 · 4 comments · May be fixed by #23132
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@WojciechMazur
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area:linting Linting warnings enabled with -W or -Xlint itype:bug regression This worked in a previous version but doesn't anymore stat:fixed in nightly This issue may be present in the latest stable or RC version of Next, but has been since fixed.

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@WojciechMazur
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WojciechMazur commented May 8, 2025 •
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Compiler version

3.7.0
3.7.1-RC1

Fixed in 3.7.2-RC1-NIGHTLY

Bisect points it was fixed in 32defdf

Minimized code

//> using options -Wunused:all 

trait MonadThrow[F[_]]
extension [T](value: T) def liftTo[F[_]](using MonadThrow[F]): F[T] = ???

trait Api[F[_]]:
  def setMaxAge(value: Int): F[Unit]
  def truncate(): F[Unit]

trait MetaStreamsSyntax:
  extension [F[_]](ms: Api[F])(using MonadThrow[F]) // warns: 
    def withMaxAge(): F[Unit] = 
      val lifted = 0.liftTo[F] // uses MonadThrow
      assert(lifted != null) // to enusre this val is used
      assert(ms != null) // to ensure ms is used
      ???
      
    def truncated(): F[Unit] = 
      ms.truncate() // required to trigger warning, does not warn if ms is not used

Output

[warn] ./example.scala:11:38
[warn] unused implicit parameter
[warn]   extension [F[_]](ms: Api[F])(using MonadThrow[F])
[warn]

Expectation

Should not warn. Bisected fix should be backported to 3.7.1-RC2 and regression test should be added
@WojciechMazur WojciechMazur added itype:bug regression This worked in a previous version but doesn't anymore area:linting Linting warnings enabled with -W or -Xlint stat:fixed in nightly This issue may be present in the latest stable or RC version of Next, but has been since fixed. labels May 8, 2025
@som-snytt
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I'll take a second look, as it should not warn in either location (for any type).

@xuwei-k
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xuwei-k commented May 9, 2025

https://github.com/sbt/sbt/blob/84ebcd3c34b09464333a8ad1ef9696b2494b9a30/tasks-standard/src/main/scala/sbt/std/TaskExtra.scala#L208-L216

Maybe same issue in sbt 2.x with Scala 3.7.0

[warn] -- [E198] Unused Symbol Warning: sbt/tasks-standard/src/main/scala/sbt/std/TaskExtra.scala:208:10 
[warn] 208 |  )(using streams: Task[TaskStreams[Key]], key: Task[?] => Key) {
[warn]     |          ^^^^^^^
[warn]     |          unused implicit parameter
[warn] -- [E198] Unused Symbol Warning: sbt/tasks-standard/src/main/scala/sbt/std/TaskExtra.scala:208:43 
[warn] 208 |  )(using streams: Task[TaskStreams[Key]], key: Task[?] => Key) {
[warn]     |                                           ^^^
[warn]     |                                           unused implicit parameter

@som-snytt
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On first glance, I missed that there is a second extension method in the "collective".

That is really just a second method that takes the "shared" parameter, and it is unused there.

This works as expected. The latest lint will not warn about a "marker" trait, however, so MonadThrow must have a member to demonstrate the warning.

@som-snytt
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som-snytt commented May 9, 2025 •
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I'll submit a PR so that the message looks like

-- [E198] Unused Symbol Warning: tests/warn/i23125.scala:13:37 -----------------
13 |  extension [F[_]](ms: Api[F])(using MonadThrow[F]) // no warns:
   |                                     ^
   |                 unused implicit parameter in extension method truncated
1 warning found

That is for any extension method; my first idea was to add words when the param is on a different line from the owner method, but I was lazy.

#23132

@som-snytt som-snytt linked a pull request May 9, 2025 that will close this issue
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area:linting Linting warnings enabled with -W or -Xlint itype:bug regression This worked in a previous version but doesn't anymore stat:fixed in nightly This issue may be present in the latest stable or RC version of Next, but has been since fixed.
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Mention extension in unused param warning som-snytt/dotty
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@som-snytt @xuwei-k @tgodzik @WojciechMazur