-Wunused:params don't work for unimplemented methods

[gatisnolv]

Reproduction steps

Scala version: 2.13.8

  object Example {
    def unusedImplicit(implicit i: Int) = ???

    def unusedExplicit(i: Int) = ???

    def unusedContextBound[A: Ordered] = ???
  }

with the compiler option
"-Wunused:params"

(or with "-Wunused:explicits", "-Wunused:implicits", "-Wunused:synthetics")

Problem

I expect the unused implicit, explicit and context bound parameters to be reported as 'never used' for each method respectively, but there are no warnings for these unimplemented methods (so the specific example in the implementing PR does not actually produce warnings, as those methods are also unimplemented with ???). If the methods are implemented to e.g. return 1, the expected warnings are produced.

[SethTisue]

@som-snytt I feel like I'm vaguely remembering you did this intentionally?

[som-snytt]

Yes, that is a long-time feature that was recently augmented under #11856

Maybe when the compiler options page is refreshed, linting behavior could be expanded upon, or I guess the word is expatiated. Although there lints that can be selected, the general goal is to avoid many knobs and levers, and find a sweet spot that is useful but not noisy, so that Xlint can be ordinarily enabled.

Maybe someone will get back to scala/docs.scala-lang#1711

[gatisnolv]

@som-snytt So just to clarify - it's intended behavior, that in the case of unimplemented methods (with '???') these parameters are not being reported as unused, correct?

[som-snytt]

Yes, that is a long-time feature that was recently augmented under #11856

[SethTisue]

The design justification is that ??? indicates explicit intent on the part of the user not to fill in an implementation. In general, warnings aim to catch accidents, not obstruct willful action.

If that design decision doesn't suit your codebase's needs, consider adding a Scalafix rule to your build (or use Wartremover or whatever) that complains if ??? is ever used at all.

[gatisnolv]

Thanks, all clear now.