Snippet illustrating how the object-protected modifier could cause the Scala compiler to generate code that throws ClassFormatError.

ObjectProtected.scala

/*
Code for illustrative purposes only, to demonstrate a particular behavior.
I already know that it doesn't run (throws ClassFormatError), and I wouldn't
write it this way to begin with :)
 */

package eg

class Base() {
  def foo = "bar"
}

trait Behavior {
  val config = "base"
}

class Derived extends Base with Behavior {
  /*
  With the object-protected modifier the compiler generates code that causes a ClassFormatError to be thrown at
  run time.
   */
  private[this] val config = "derived"
  /*
   Changing the modifier to private yields a compile error.
   */
//  private val config = "derived"
}

object Main extends App {
  val instance = new Derived
  instance.foo
  println("done")
}