scala – 在抽象构造函数中访问overriden val时出现NullPointerE

abstract class Bar[T] {
    val f: PartialFunction[T,T]
    val default: PartialFunction[T,T] = { case x => x }
    val chained = f orElse default
}

class Foo extends Bar[Int] {
    val f: PartialFunction[Int,Int] = { case 1 => 2 }
}

看着它崩溃：

scala> val foo = new Foo
java.lang.NullPointerException
        at Bar.<init>(<console>:8)
        at Foo.<init>(<console>:6)
        at .<init>(<console>:7)
        at .<clinit>(<console>)
        at RequestResult$.<init>(<console>:9)
        at RequestResult$.<clinit>(<console>)
        at RequestResult$scala_repl_result(<console>)
        ....

但是,如果我们将链接放在具体类中：

abstract class Bar[T] {
    val f: PartialFunction[T,T] = { case x => x }
}

class Foo extends Bar[Int] {
    val f: PartialFunction[Int,Int] = { case 1 => 2 }
    val chained = f orElse default
}

它按预期工作：

scala> val foo = new Foo
foo: Foo = Foo@16132c4

我不得不承认我完全不知道这里发生了什么.错误？ (这是在Scala 2.8.1上.)