2.12.4 regression. "-Ywarn-unused" reports unused implicit param, however it's used within macro expansion

[mosyp]

Given scalacOptions ++= Seq("-Xfatal-warnings", "-Ywarn-unused") and simple macro:

object TestMacro {
  def runAsync[T](x: T): Future[T] = macro runAsyncMacro[T]
  def runAsyncMacro[T](c: blackbox.Context)(x: c.Tree)(implicit t: c.universe.WeakTypeTag[T]): c.Tree = {
    import c.universe._
    c.typecheck(q"""scala.concurrent.Future[$t]($x)""")
  }
}

Following code compiles in 2.12.3:

object TestRun {
  def test(implicit ec: ExecutionContext): Unit = {
    TestMacro.runAsync(123)
  }
}

But fails in 2.12.4:

[error] TestRun.scala:4: parameter value ec in method test is never used
[error]   def test(implicit ec: ExecutionContext): Unit = {

[SethTisue]

caused by scala/scala#6096, I presume. so this is working as intended, but also there's room for further improvement, e.g. by making it possible for a macro author to control which behavior is intended. @som-snytt is there a ticket of record on that?

[som-snytt]

Duplicates #10571

use -Ywarn-macros:after to lint after expansion.

[mxl]

@som-snytt Thanks! Where can I read more about these scalac options?

[som-snytt]

@mxl That's a good question. I haven't been able to turn on the optimizer for a while now.

scalac -help -X -Y, -opt:help. The release notes also include handy links to bugs and PRs.

I just realized I forgot to update my shell alias to scala -Dscala.repl.welcome=banner.