已删除：private[this] 和 protected[this]

private[this] 和 protected[this] 访问修饰符已经被弃用，将被逐步淘汰。

以前，这些修饰符需要用来

• 避免生成 getter 和 setter
• 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).

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.

---