Question

I’m having difficulties with the private annotation of inner class methods and constructors. While this works as expected:

trait A {
  protected def lala = ()
}

trait B extends A {
  lala
}

The following doesn’t:

trait A {
  class Lala protected()
}

trait B extends A {
   new Lala
}

Neither does this:

trait A {
  class Lala private[A]()
}

trait B extends A {
  new Lala
}

The only way around is something like this:

object Screwed {
  trait A {
    class Lala private[Screwed]()
  }

  trait B extends A {
    new Lala
  }
}

Does Scala really fail here to provide a clear mechanism, or am I missing something? My guess would have been that it should be private[this.type] but scalac doesn’t want to swallow that at all…

Answer 1

Well,

trait A {
  class Lala protected()
  new Lala
}

doesn’t compile either. The error message seems pretty reasonable:

 error: constructor Lala in class Lala cannot be accessed in trait A
 Access to protected constructor Lala not permitted because
 enclosing class trait A in object $iw is not a subclass of
 class Lala in trait A where target is defined

Protected access means you can only access that constructor from that class or sub-classes. You’re trying to call it from the enclosing trait. One thing you can do is this:

trait B extends A {
  class Gaga extends Lala
  new Gaga
}

I wouldn’t expect your third example to compile, for similar reasons.

Also note the difference between a protected constructor and a protected class. So for example:

trait A {
  protected class P
  class U  // unprotected
}

class B extends A {
  new P  // OK
  new U  // OK
}

val b = new B
new b.P  // error
new b.U  // OK