FboundedParsedHeaders.scala

sealed trait HeaderKey {
  type HeaderT <: Parsed[_]

  def go(t: HeaderT): Unit = ()

  def name: String
}

/*
trait Header {
  def name: String
}
*/

trait Parsed[T <: Parsed[T]] { self: T =>
  final type MyHeaderKey = HeaderKey { type HeaderT = T }
  def key: MyHeaderKey
  def name: String= key.name
  def parsed: this.type = this
}

object ThisKey extends HeaderKey {
  type HeaderT = ThisHeader
  def name = "ThisHeader"
}

case class ThisHeader(value: String) extends Parsed[ThisHeader] {
  def key = ThisKey
}

object OtherKey extends HeaderKey {
  type HeaderT = OtherHeader
  def name = "OtherHeader"
}

case class OtherHeader(value: String) extends Parsed[ThisHeader] {
  // def key = ThisKey // Fails to compile!!!
  def key = OtherKey
}

object Main {
  def main(args: Array[String]): Unit = {
    println("Hello, world!")
  }
}