aaa.scala

trait User {
  val name: String
}

trait Password {
  val key: Option[String]
}

//-------------------------------

trait NoPassword extends Password {
  val key = None
}

//-------------------------------

trait PublicUser {
  self: User with Password =>
}

trait PrivateUser extends NoPassword {
  self: User with Password =>
}

//-------------------------------

val publicUser = new PublicUser with User with Password {
  val name = "Hoge"
  val key = Some("aaa")
}

val privateUser = new PrivateUser with User with NoPassword {
  val name = "Huga"
}