I don't know about the serialization issue but I don't doubt you had it.
> def ++[B >: A, That](that: GenTraversableOnce[B])(implicit bf: CanBuildFrom[IndexedSeq[A], B, That]): That
I should point out that this is the Scala 2.12 and and earlier signature. `CanBuildFrom` is gone from the Scala collections since Scala 2.13. In fact, the collections were redesigned for Scala 2.13 primarily to simplify method signatures, following community feedback.
> def ++[B >: A, That](that: GenTraversableOnce[B])(implicit bf: CanBuildFrom[IndexedSeq[A], B, That]): That
I should point out that this is the Scala 2.12 and and earlier signature. `CanBuildFrom` is gone from the Scala collections since Scala 2.13. In fact, the collections were redesigned for Scala 2.13 primarily to simplify method signatures, following community feedback.