"Internal error: unable to find the outer accessor symbol of ..." when deriving instance for a nested type

[notxcain]

Version: 1.12

Reproducer: https://scastie.scala-lang.org/lZwX8BQSThWjL8sEAnSPnw

[adamw]

Seems to be a problem with a combination of classes defined in traits + default field values

[notxcain]

Yes, but it works if you put derivation inside the companion.

trait X {
  case class F(i: Int = 5)
  object F {
    implicit def tc: TC[F] = TC.derive[F]
  }
}