This definition works with mongodb case class macros:

sealed class Tree
final case class Node(left: Tree, right: Tree) extends Tree
final case class Leaf(x: Int) extends Tree

But to serialize it with other serializers (avro4s) it should be like this instead - a trait:

sealed trait Tree // a trait instead of class
final case class Node(left: Tree, right: Tree) extends Tree
final case class Leaf(x: Int) extends Tree

But this code isn't supported if we use mongodb macros, failing with compile time error "No constructor unsupported class type". From my understanding the macro at this point already found subclasses and needs a constructor, which trait doesn't have. Don't know why the constructor is needed here, because the main class will be never instantiated.

We have a pretty painful workaround to use them both - extending both class and trait:

sealed trait TreeLike
sealed class Tree
final case class Node(left: TreeLike, right: TreeLike) extends Tree
final case class Leaf(x: Int) extends Tree

A TreeLike codec provider is still needed to lookup the Tree codec in the registry. And it makes still less work compared to implementing all the codecs manually, which we already did earlier.

Would be much better if macro would support traits as well. Is it possible?