，则隐式扩展 A 。您所做的是语法糖，等效于此：

With this, you implicitly extend A. What you did is syntactic sugar, equivalent to this:

class A' extends A {
    val a = 3
}
val a = new A'

这些括号仅允许您扩展a类，创建一个新的匿名类，该类实例化值 a ，因此不再抽象。

These brackets merely let you extend a class on the fly, creating a new, anonymous class, which instantiates the value a and is therefore not abstract any more.