Unexpected difference in behavior seemingly due to using private[this] instead of private

Question

These two classes behave differently; the cause seems related to the use of a private[this] declaration instead of a private. Can somebody explain why, please?

• private:

  class Person(
    private var _age: Int
  ) {
    if (_age < 0) { _age = 0 }
  
    def age = _age
    def age_=(newAge: Int) {
      if (newAge > _age) { _age = newAge }
    }
  }
  

  In the REPL, this behaves as I was expecting for both classes; that is, the age method gets _age, which has been set to the appropriate value during construction:

  scala> val person = new Person(-1)
  person: Person = Person@200a570f
  
  scala> person.age
  res0: Int = 0
  

• private[this]:

  class Person(
    private[this] var _age: Int
  ) {
    if (_age < 0) { _age = 0 }
  
    def age = _age
    def age_=(newAge: Int) {
      if (newAge > _age) { _age = newAge }
    }
  }
  

  In the REPL, person.age appears to take the value of _age prior to the evaluation of the if expression. It works as expected after using the setter, though:

  scala> val person = new Person(-1)
  person: Person = Person@6f75e721
  
  scala> person.age
  res0: Int = -1
  
  scala> person.age = 0
  person.age: Int = 0
  
  scala> person.age
  res1: Int = 0
  

Answer 1

This is a known bug, namely SI-6880.