Nothing is - together with scala.Null - at the bottom of Scala's type hierarchy.
Scala中的Nothing和Null两个类位于Scala类型继承树的底部。
Nothing is a subtype of every other type (including scala.Null); there exist no instances of this type.
Nothing是Scala中其他任何类（包含Null）的子类，Nothing不能再被子类化。
Although type Nothing is uninhabited, it is nevertheless useful in several ways.
Nothing在几个方面发挥作用。
For instance, the Scala library defines a value scala.collection.immutable.Nil of type List[Nothing].
比如，Scala用Nil定义空集合，而Nil其实是List[Nothing]
Because lists are covariant in Scala, this makes scala.collection.immutable.Nil an instance of List[T], for any element of type T.
因为Scala中的列表是协变的，所以Nil是任何泛型列表List[T]的实例，而只有Nothing是所有类型T的子类，所以在
定义Nil的时候，Nothing就发挥了自己的用途。
Another usage for Nothing is the return type for methods which never return normally. One example is method error in scala.sys, which always throws an exception.
Nothing也被用在当方法不正常返回的场合。比如sys.error("err")。它的源码是：
def error(message: String): Nothing = throw new RuntimeException(message)
这个方法不会返回Unit，因为它不是正常结束，是异常中断了。