Which things around case classes will be removed after Scala 2.9 exactly?

问题

I know that the are some changes planned regarding case classes, like disallowing inheritance between them:

scala> case class Foo()
defined class Foo

scala> case class Bar() extends Foo()
<console>:9: warning: case class `class Bar' has case ancestor `class Foo'.  Case-to-case inheritance has potentially dangerous bugs which are unlikely to be fixed.  You are strongly encouraged to instead use extractors to pattern match on non-leaf nodes.
       case class Bar() extends Foo()
                  ^
defined class Bar

or case classes without parameter list (not sure about that):

scala> case class Foo
<console>:1: warning: case classes without a parameter list have been deprecated;
use either case objects or case classes with `()' as parameter list.
       case class Foo
                     ^
<console>:7: warning: case classes without a parameter list have been deprecated;
use either case objects or case classes with `()' as parameter list.
       case class Foo
                     ^
defined class Foo

What else is currently @deprecated?

回答1:

Every class in Scala must have at least one non-implicit parameter section. If you don't include one, the compiler will add it for you.

scala> class X
defined class X

scala> new X()
res4: X = X@68003589

scala> class Y
defined class Y

scala> new Y()
res5: Y = Y@467f788b

Case classes are no exception. But the interaction with pattern matching is a source of confusion and bugs, which motivates the deprecation.

scala> case class A
<console>:1: warning: case classes without a parameter list have been deprecated;
use either case objects or case classes with `()' as parameter list.
       case class A
                   ^
defined class A

scala> val a = A()
a: A = A()

scala> (a: Any) match { case A => 1; case _ => 2 }
res0: Int = 2

scala> val companion = A
companion: A.type = A

scala> (companion: Any) match { case A => 1; case _ => 2 }
res0: Int = 1

As Dean suggests, it's usually better to model this with a case object.

I'm not aware of a timeline for removing support for empty-param list case classes. Case class inheritance was almost removed in 2.9.0, but that has been deferred until the next major release.