Map's second parameter is Object on reflection for primitive types

[scabug]

So there I was, trying to reflect on constructor parameters in Scala 2.9.0.RC1.

class Derper(m: Map[String, Long])

class Herper(m: Map[String, Derper])

object DoAReflectThing {
  def main(args: Array[String]) {
    println(classOf[Derper].getConstructors.head.getGenericParameterTypes.head)
    println(classOf[Herper].getConstructors.head.getGenericParameterTypes.head)
  }
}

=== What is the expected behavior? ===

In Scala 2.8.1, the output is this:

scala.collection.immutable.Map<java.lang.String, java.lang.Long>
scala.collection.immutable.Map<java.lang.String, com.yammer.streamie.Derper>

This is awesome, since I have the information I need.

=== What do you see instead? ===

In Scala 2.9.0.RC1, this happens instead:

scala.collection.immutable.Map<java.lang.String, java.lang.Object>
scala.collection.immutable.Map<java.lang.String, com.yammer.streamie.Derper>

sad trombone noise

Now I have no idea what the values of the map should be.

This is really important for me, since I'm the author of Jerkson, a thin Scala layer on top of Jackson. I use reflection to deserialize case classes from JSON objects, but if one of the values of a case class is a Map, I don't have enough information to deserialize it in 2.9.0.RC1.

=== What versions of the following are you using? ===

• Scala: 2.9.0.RC1
• Java: 1.6.0_24
• Operating system: OS X 10.6.7

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-4403?orig=1
Reporter: Coda Hale (codahale)
See #4912

[scabug]

@paulp said:
There isn't much we can do. See #4214 among many others in the 4000s. Putting java.lang.Long there is a lie (there is no method which returns java.lang.Long) and it breaks things. Primitive types can't appear in the signature: that breaks things even worse. There is no choice left but Object.

You can (and will have to) get the information out of the scala signature. Clearly our motivation to make this easy with a scala reflection library has gone way up recently, but it's not imminent. There is code out there somewhere to dredge the info out, and/or you can look at scalap.

[scabug]

@retronym said:
I hope the API evolves to make this more convenient (Type#paramss is conspicuously absent from the public API, which is why we have to pattern match below), but reflection will allow for this:

scala> class Derper(m: Map[String, Long])
defined class Derper

scala> val tpe = reflect.mirror.classToType(classOf[Derper])
tpe: reflect.mirror.Type = Derper

scala> val c = tpe.member(reflect.mirror.nme.CONSTRUCTOR)
c: reflect.mirror.Symbol = constructor Derper

scala> val reflect.mirror.MethodType(Seq(param0), _) = c.asType
warning: there were 1 unchecked warnings; re-run with -unchecked for details
param0: reflect.mirror.Symbol = value m

scala> val typeArgs = param0.asType.typeArguments
typeArgs: List[reflect.mirror.Type] = List(String, Long)

scala> typeArgs.last.typeSymbol.fullName
res25: String = scala.Long

So given the replacement, and Paul's explanation of why Java reflection can never again see the full details, let's close as not-a-bug.