Scala Wart: Abstract/non-final case classes

Opening this issue, as suggested by Martin, to provide a place to discuss the individual warts brought up in the blog post Warts of the Scala Programming Language and the possibility of mitigating/fixing them in Dotty (and perhaps later in Scala 2.x). These are based on Scala 2.x behavior, which I understand Dotty follows closely, apologies in advance if it has already been fixed

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You can inherit from case classes and extend them with new functionality:

@ case class Foo(i: Int)
defined class Foo

@ Foo(1)
res18: Foo = Foo(1)

@ Foo(1).i
res19: Int = 1

@ class Bar extends Foo(1)
defined class Bar

@ (new Bar).i
res21: Int = 1

You can even declare it an abstract case class to force someone to inherit from it rather than instantiating it directly. If you want your case class to not allow inheritance you should label it final

As far as I can tell, “nobody” does any of this: people don’t inherit from case classes, declare their case classes abstract, or rememebr to mark them final. Literally the only programmer I’ve ever seen making good use of abstract case classes and inheritance is probably Martin Odersky himself. I think we should disallow it, and just force people to use normal classes if they want to inherit from them.

[fommil]

generally I don’t think the compiler should be responsible for warts and linting. The compiler should either allow or disallow certain features that are planned to be removed, with a flag to ignore the warning, allowing a grace time for users to migrate. Scalafix is proving itself to be an excellent tool in this regards, allowing the compiler to be simpler.

[fommil]

given the usecase in scalaz, I’m sure having a record of how this works for posterity would be good.

compiler development seems to be an endless amount of moving people to other services or rooms to have a conversation. If I ever write a compiler, I’ll be sure to spend a good amount of time setting up at least 5 mailing lists, gitter rooms, github accounts, discourse sites and twitter accounts… just so I can do a basic job of sending people to the right place.