在Scala中匹配案例类的子类

Question

Why does this fail to compile (or work?):

  case class A(x: Int)
  class B extends A(5)

  (new B) match {
    case A(_) => println("found A")
    case _ => println("something else happened?")
  }

The compiler error is:

constructor cannot be instantiated to expected type;  found   : blevins.example.App.A  required: blevins.example.App.B

Note that this compiles and runs as expected:

  (new B) match {
    case a: A => println("found A")
    case _ => println("something else happened?")
  }

---

ADDENDUM

Just for reference, this compiles and runs fine:

  class A(val x: Int)
  object A {
    def unapply(a: A) = Some(a.x)
  }
  class B extends A(5)

  (new B) match {
    case A(i) => println("found A")
    case _ => println("something else happened?")
  }

Answer 1

This works, at least in 2.8:

scala>   case class A(x: Int)                           
defined class A

scala>   class B extends A(5)                           
defined class B

scala>   (new B: A) match {                             
     |     case A(_) => println("found A")              
     |     case _ => println("something else happened?")
     |   }                                              
found A

I haven't found the a pointer to the particular bug that causes the original problem, but ignore the warnings about case class inheritance at your own peril.