Scala: How to make a field visible for subclasses, but not visible for their inner classes?

Question

How can I make a field visible for subclasses, but not visible for their inner classes?

class Foo

class Parent {
  protected val x = 5
} 

class Sub extends Parent {
  println(x) // 5

  new Foo {
    println(x) // it's 5, but I want it to be a compiler error...
  }
}