extractor pattern で skolem 型変数の扱いが意味わからんという話。

patmat_bug_extractor_with_skolem.scala

import scala.language.higherKinds

sealed trait Base[F[_]] { // all compilation is succeeded if F' kind is *
  def a: String
}
case class Ctor[F[_]](a: String) extends Base[F]
abstract class Etor[F[_]] extends Base[F] {
  def f(q: Base[F]): String = q match {
    case Ctor(fa) => fa // NP
    case Etor(fa) => fa // error! why???
    // exractor_with_skolem.scala:10: error: pattern type is incompatible with expected type;
    // found   : Etor[F(in method unapply)]
    // required: Base[F(in class Etor)]
    // case Etor(fa) => fa
    //          ^

    // case etorr: Etor[F] => etor.a // NP 
  }
}
object Etor {
  def unapply[F[_]](node: Etor[F]): Option[String] = {
    if (node eq null) None else Some(node.a)
  }
}