BadOptions.scala

import scopt.OptionParser

case class BaseConfig(host: String, port: Int)


trait BaseParser { self: OptionParser[BaseConfig] =>
  opt[String]("host") action { (x, c) =>
    c.copy(host = x)
  } text "Hostname of smrt server"

  opt[Int]("port") action { (x, c) =>
    c.copy(port = x)
  } text "Services port on smrt server"
}


object BaseProgram {
  lazy val parser = new OptionParser[BaseConfig]("basic-config") with BaseParser {
    head("This is the basic program", "0.0.1")
  }
}

case class ExtendedConfig(host: String, port: Int, uiPort: Int)

trait ExtendedParser { self: OptionParser[ExtendedConfig] =>
  opt[Int]("ui-port") action { (x, c) =>
    c.copy(uiPort = x)
  } text "GUI port on smrt server"
}

object ExtendedProgram {
  lazy val parser = new OptionParser[ExtendedConfig]("extended-config") with BaseParser with ExtendedParser {
    head("This is the more fully-featured program")
  }
}

This fails:

BadOptions.scala:33: illegal inheritance;
[error]  self-type scopt.OptionParser[ExtendedConfig] with BaseParser with ExtendedParser does not conform to BaseParser's selftype BaseParser with scopt.OptionParser[BaseConfig]
[error]   lazy val parser = new OptionParser[ExtendedConfig]("extended-config") with BaseParser with ExtendedParser {
[error]                                                                              ^