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unused annotation does not silence warning in given using with shorthand #23033

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eejbyfeldt opened this issue Apr 23, 2025 · 3 comments · May be fixed by #23037
Open

unused annotation does not silence warning in given using with shorthand #23033

eejbyfeldt opened this issue Apr 23, 2025 · 3 comments · May be fixed by #23037
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@som-snytt
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area:annotations area:implicits related to implicits area:linting Linting warnings enabled with -W or -Xlint itype:bug regression This worked in a previous version but doesn't anymore

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@eejbyfeldt
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Compiler version

3.7.0-RC3

Minimized code

import scala.util.NotGiven
import scala.annotation.unused

object Test {
  given [T](using @unused ev: NotGiven[T <:< Int]): AnyRef with {}
}

Output

Compiled using -Wunused:all

-- [E198] Unused Symbol Warning: unused_notgiven_annotation.scala:5:26 -----------------------
5 |  given [T](using @unused ev: NotGiven[T <:< Int]): AnyRef with {}
  |                          ^^
  |                          unused implicit parameter
1 warning found

Expectation

@unused should silence the warning as it does in other cases.

For example changing the code to

import scala.util.NotGiven
import scala.annotation.unused

object Test {
  given [T](using @unused ev: NotGiven[T <:< Int]): AnyRef = new AnyRef {}
}

works as expected.
@eejbyfeldt eejbyfeldt added itype:bug stat:needs triage Every issue needs to have an "area" and "itype" label labels Apr 23, 2025
@eejbyfeldt eejbyfeldt mentioned this issue Apr 23, 2025
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@eejbyfeldt
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If #22969 get fixed the test case might need to be changed.

@som-snytt
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som-snytt commented Apr 23, 2025 •
edited
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Perhaps the annotation winds up on the wrong element in the desugaring.

    given class given_AnyRef[T >: Nothing <: Any](using
      ev: scala.util.NotGiven[<:<[T, Int]]) extends AnyRef() {
      T
      @unused protected given val ev: scala.util.NotGiven[T <:< Int]
    }
    final given def given_AnyRef[T >: Nothing <: Any](using
      @unused ev: scala.util.NotGiven[<:<[T, Int]]): Test.given_AnyRef[T] =
      new Test.given_AnyRef[T](using ev)()

or it is dropped in posttyper.

It's time to revisit this condition:
https://github.com/scala/scala3/blame/main/compiler/src/dotty/tools/dotc/transform/PostTyper.scala#L249

@som-snytt som-snytt linked a pull request Apr 23, 2025 that will close this issue
Open
@Gedochao Gedochao added area:annotations area:implicits related to implicits area:linting Linting warnings enabled with -W or -Xlint regression This worked in a previous version but doesn't anymore stat:needs bisection Need to use nightly builds and git bisect to find out the commit where this issue was introduced and removed stat:needs triage Every issue needs to have an "area" and "itype" label labels Apr 24, 2025
@Gedochao
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Last good release: 3.7.0-RC1-bin-20250127-89c20f8-NIGHTLY
First bad release: 3.7.0-RC1-bin-20250128-9cb97ec-NIGHTLY
c65d45d is the first bad commit (#20894)

@Gedochao Gedochao removed the stat:needs bisection Need to use nightly builds and git bisect to find out the commit where this issue was introduced label Apr 24, 2025
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Keep unused annot on params som-snytt/dotty
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@som-snytt @Gedochao @eejbyfeldt