When are forward references compile-time errors?

Question
1

I came across a situation where the following was not a compile time error, but did give rise to a runtime error unless I reordered the definition of port and uriScheme (I’ll note that IntelliJ did warn me of a “suspicious forward reference”):

  implicit val port: Rx[Port] = (currentApiUri |@| uriScheme).map{ (curUri, curSchm) =>
    Option(curUri.getPort) match {
      case Some(prt) => if (prt < 0) Port(schemeToDefaultPort(curSchm.scheme)) else Port(prt)
      case None => Port(defaultPort)
    }
  }
  implicit val uriScheme: Rx[UriScheme] = currentApiUri.map{ curUri =>
    Option(curUri.getScheme) match {
      case Some(scheme) => UriScheme(scheme)
      case None => UriScheme(defaultScheme)
    }
  }

Is there a scalac flag that will not generate additional code, but will turn such non-lazy forward references into compile-time errors?