scala 2.12.4 false positive warning on unused private implicit val

[yanns]

Scala 2.12.4 emits a new warning (compared to scala 2.12.3) that is false-positive.

[warn] <path>/src/main/scala/Schema.scala:13:24: private val output in object Schema is never used
[warn]   private implicit val output = deriveObjectType[Ctx, Output]()
[warn]                        ^
[warn] one warning found

This appears with a implicit value used by code generated by a macro.

To reproduce the issue:
https://github.com/yanns/scala-2.14-unused-val

[hrhino]

This appears to be the same as #10571; there's some discussion in that ticket.

[yanns]

Thanks for the info!!

Using -Ywarn-macros:after fixes the problem.