Dropped: private[this] and protected[this]

The private[this] and protected[this] access modifiers will be deprecated and phased out. Migration warning will be shown in Scala 3.4+.

Previously, these modifiers were needed for

• avoiding the generation of getters and setters
• excluding code under a private[this] from variance checks. (Scala 2 also excludes protected[this] but this was found to be unsound and was therefore removed).
• avoiding the generation of fields, if a private[this] val is not accessed by a class method.

The compiler now infers for private members the fact that they are only accessed via this. Such members are treated as if they had been declared private[this]. protected[this] is dropped without a replacement.

This change can in some cases change the semantics of a Scala program, since a private val is no longer guaranteed to generate a field. The field is omitted if

• the val is only accessed via this, and
• the val is not accessed from a method in the current class.

This can cause problems if a program tries to access the missing private field via reflection. The recommended fix is to declare the field instead to be qualified private with the enclosing class as qualifier. Example:

  class C(x: Int):
    private[C] val field = x + 1
      // [C] needed if `field` is to be accessed through reflection
    val retained = field * field