TypeTag implicit incorrectly marked as unused

[koterpillar]

reproduction steps

using Scala 2.13.5 with -Wunused:

import scala.reflect.runtime.universe._

object foo {
  case class Wrap[A](value: A)

  def withTag[A: TypeTag]: Unit = ()

  // new in 2.13.5, incorrect:
  // warning: parameter value evidence$2 in method withTagWrap1 is never used
  def withTagWrap1[A: TypeTag]: Unit = withTag[Wrap[A]]

  // this is expected:
  // error: No TypeTag available for blah.foo.Wrap[A]
  def withTagWrap2[A]: Unit = withTag[Wrap[A]]
}

problem

The TypeTag[A] context bound in withTagWrap1 is used (to derive TypeTag[Wrap[A]]) and should not be reported as unused.

The same code works in Scala 2.13.4.

Possibly related to scala/scala#9346.

[som-snytt]

It won't warn with -Wmacros:after. Use -Xprint:typer to see the expansion.

Use -Vmacro-lite to see

performing macro expansion scala.reflect.api.`package`.materializeTypeTag[foo.Wrap[A]]

It's possible to disable -Wunused:-synthetics or use -Xlint:unused for less fuss, or use -Wconf to limit warnings.

Thanks for the report. Probably at this point, the interaction with macros will not be further improved.

[NthPortal]

@som-snytt why would it have to do with -Wmacros:after? there are no macros here

[Jasper-M]

@NthPortal There's an implicit TypeTag[A] in scope, but a (intrinsic) macro is required to generate the TypeTag[Wrap[A]].