score:4

Accepted answer

You are probably trying to cast across class loaders. You can't do this--each class loader maintains its own hierarchy (for those classes not passed off to a common parent loader). Try calling getClassLoader on both your returned map and on a freshly-created one.

Incidentally, Map$Map2 is just an implementation detail--a subclass of Map for dealing with two-element maps. It casts just fine normally:

scala> val m = Map(1->"one", 2 -> "two"): Object
m: Object = Map(1 -> one, 2 -> two)

scala> m.getClass
res0: Class[_ <: Object] = class scala.collection.immutable.Map$Map2

scala> m.asInstanceOf[scala.collection.immutable.Map[Int,String]]
res1: scala.collection.immutable.Map[Int,String] = Map(1 -> one, 2 -> two)

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