How to copy a case class and add a mixin?

Question

I'd like to have a base case class I can extend at will into different types. I had something like this working previously, but I must have screwed something up, because now I'm getting compile errors.

trait JobLike case class Task(name: String) { def as[T <: JobLike]: Task with T = new Task(this.name) with T }

The problem is, this gives me a compiler error like this:

java.lang.UnsupportedOperationException: addChild inapplicable for type T at scala.reflect.internal.Symbols$Symbol.addChild(Symbols.scala:1835) at scala.tools.nsc.typechecker.Namers$Namer.$anonfun$templateSig$1(Namers.scala:1119) at scala.tools.nsc.typechecker.Namers$Namer.templateSig(Namers.scala:1107) at scala.tools.nsc.typechecker.Namers$Namer.classSig(Namers.scala:1178) at scala.tools.nsc.typechecker.Namers$Namer.memberSig(Namers.scala:1788) at scala.tools.nsc.typechecker.Namers$Namer.typeSig(Namers.scala:1751) at scala.tools.nsc.typechecker.Namers$Namer$MonoTypeCompleter.completeImpl(Namers.scala:836) ... at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:82) at scala.tools.nsc.MainGenericRunner.run$1(MainGenericRunner.scala:85) at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:96) at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:101) at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)</code>

I swear, I had something working yesterday that basically did just this, but now it's throwing this error.

I can't find a compiler version where this successfully compiles but normally it should fail with T is not a class type so this looks like a kind of regression. Consider filing a bug report at github.com/scala/bug/issues — Jasper-M
– Jasper-M, Commented Aug 25, 2017 at 19:48

HTNW · Accepted Answer · 2017-08-25 19:53:36Z

You managed to hit a compiler bug! While that's certainly an issue, and should be reported, the thing is that your code isn't possible anyway, and it's better that it's crashing and bringing you here than "working" and doing something horribly wrong. Because T can be any subtype of JobLike, there's simply no way to mix it in at compile time. I could very well do trait Boom extends JobLike { def something: Int }; task.as[Boom], and then task couldn't create a Boom because there's no implementation for something.

You can provide asJobLike: Task with JobLike instead of a general as[T]: Task with T, which is completely possible. (Note that it's discouraged to make subtypes of case classes, so you may want to create a new class. The reason is that it breaks the reflexivity of equals.)

Something rather filthy that you can do is call (new Task(this.name) with JobLike).asInstanceOf[Task with T]. asInstanceOf only checks the cast for the leftmost type in the with, so this will not ClassCastException off the bat (you can also do "a".asInstanceOf[String with Int].). It will generally sorta work (modulus asInstanceOf[T] being false) as long as T has no members.

Something horrible that you can also do is use runtime reflection to dynamically create the required class. The trick as a whole is filthy and completely unsafe, so I won't say it here, but have a link.

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