Hello,

I was just wondering whether this has ever been discussed for changing in the new implementation, sorry if it has been, but I can't find an issue:

final case class ::[B](override val head: B, private[scala] var tl: List[B]) extends List[B]

That var makes List a thread unsafe data-type, breaking users expectations, because it's only final values that guarantee that that tl won't be seen as null after the constructor of :: has finished. Users imo expect final values and thread-safety. The ListBuffer trick is cool, but it's not worth it.