Collection encoder uses MapObjects, this does not have special logic for Vector and will return a Seq that can be a different class.

Scope is serialisation with interpretation (many use-cases will do codegen), compilation of MapObjects is correct and uses builders. Eval of MapObjects with NewInstance fails as NewInstance will look for a Seq constructor parameter but only a Vector parameter exists.

In fixing #803 the use of Vector in UdfTests."one argument udf" failed as:

Message: scala.collection.immutable.Stream$Cons cannot be cast to scala.collection.immutable.Vector

which is true. The Stream$Cons is created by MapObjects:

  private lazy val mapElements: scala.collection.Seq[_] => Any = customCollectionCls match {
    case Some(cls) if classOf[WrappedArray[_]].isAssignableFrom(cls) =>
      ...
    case Some(cls) if classOf[scala.collection.Seq[_]].isAssignableFrom(cls) =>
      // Scala sequence
      executeFuncOnCollection(_).toSeq

There are a number of these potential issues with all similar Seq derived types (Set derived as well).

The inbuilt ScalaReflection createIterableEncoder also correctly uses the appropriate builder.