Why protected inner class of object may be seen outside of its definition scope?

Question

Consider the following:

scala> :paste
// Entering paste mode (ctrl-D to finish)

object O {
  protected case class I(x: Int)
  trait T {
    protected def m: I = I(0)
  }
}

val i = new O.T { override def m = super.m }.m

// Exiting paste mode, now interpreting.

defined object O
i: O.I = I(0)

scala> :type i
O.I

If I add : O.I after val i this code snippet fails to compile, but statement i.x compiles and returns 0 in runtime.

Is it a compiler bug or there is a good reason for such behavior?

Answer 1

Derived classes can in any case brake protection as they have full access to base class protected members. Consider public method foo below

val i = new O.T {
  override protected def m = super.m
  def foo = m
}

We keep m as protected, nevertheless i.foo indirectly "brakes out" of protection and evaluates to res0: O.I = I(0). So it seems we do not gain much by preventing widening on overriding.

Also consider related answer to When overriding a method, why can I increase access but not decrease it?:

It's a fundamental principle in OOP: the child class is a fully-fledged instance of the parent class, and must therefore present at least the same interface as the parent class.