Unused Local Value Warning with Scala 2.12.4 when Using (Shapeless) Tagged Types

[michaelahlers]

Maybe not a ScalaCheck-Shapeless bug per se, but after switching from Scala 2.12.3 with -Xlint enabled I began getting unused value warnings for implicit Arbitrary instances for (Shapeless) tagged types in scope.

To illustrate:

// build.sbt

name := "sample"
version := "0.1"

scalaVersion := "2.12.3"
scalacOptions :=
  "-Xfatal-warnings" ::
    "-Xlint" ::
    Nil

libraryDependencies ++=
  "com.chuusai" %% "shapeless" % "2.3.3" ::
    "com.github.alexarchambault" %% "scalacheck-shapeless_1.13" % "1.1.8" % Test ::
    Nil

// src/test/scala/Unused.scala

import org.scalacheck.Arbitrary._
import org.scalacheck.ScalacheckShapeless._
import org.scalacheck._
import shapeless.tag
import shapeless.tag._

object Unused {

  sealed trait Foo
  case class Bear(s: String, i: Int @@ Foo)

  val gen: Gen[Bear] = {
    implicit val arbFoo: Arbitrary[Int @@ Foo] = Arbitrary(arbitrary[Int].map(tag[Foo].apply[Int]))
    arbitrary[Bear]
  }

}

That works fine with 2.12.3, but assigning 2.12.4 to scalaVersion and recompiling results in:

sbt:sample> ;clean;test:compile
[success] Total time: 2 s, completed Jan 28, 2018, 1:44:20 PM
sbt:sample> set scalaVersion := "2.12.4"
sbt:sample> ;clean;test:compile
[error] /tmp/sample/src/test/scala/Unused.scala:13:16: local val arbFoo in value gen is never used
[error]                 implicit val arbFoo: Arbitrary[Int @@ Foo] = Arbitrary(arbitrary[Int].map(tag[Foo].apply[Int]))
[error]                              ^

Setting -Xlint:-unused,_ gets this compiling with 2.12.4.

Can support for arbitrary tagged types be added to ScalaCheck-Shapeless?