Extracting case classes with Type Parameters fails with runtime error

Not sure if there is an issue on this, but currently you are unable to automatically extract case classes if they have type constructors, more detail is in the scala REPL below.

Is this an intended limitation, or is it a case of a feature not being supported yet?

Scala version is 2.11.1

import spire.math._
scala> case class Test(a:Natural)
defined class Test

scala> val a = Test(Natural(2))
a: Test = Test(2)

scala> Extraction.decompose(a)
res2: org.json4s.JValue = JObject(List((a,JInt(2))))

scala> case class Test2[T <% Number](b:T)
defined class Test2

scala> val b = Test2(Natural(3))
b: Test2[spire.math.Natural] = Test2(3)

scala> Extraction.decompose(b)           
org.json4s.package$MappingException: Can't find constructor for Test2
  at org.json4s.reflect.ScalaSigReader$.readConstructor(ScalaSigReader.scala:21)
  at org.json4s.reflect.Reflector$ClassDescriptorBuilder.ctorParamType(Reflector.scala:90)
  at org.json4s.reflect.Reflector$ClassDescriptorBuilder$$anonfun$constructorsAndCompanion$3$$anonfun$13.apply(Reflector.scala:148)
  at org.json4s.reflect.Reflector$ClassDescriptorBuilder$$anonfun$constructorsAndCompanion$3$$anonfun$13.apply(Reflector.scala:122)
  at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:245)
  at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:245)
  at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59)
  at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48)
  at scala.collection.TraversableLike$class.map(TraversableLike.scala:245)
  at scala.collection.AbstractTraversable.map(Traversable.scala:104)
  at org.json4s.reflect.Reflector$ClassDescriptorBuilder$$anonfun$constructorsAndCompanion$3.apply(Reflector.scala:122)
  at org.json4s.reflect.Reflector$ClassDescriptorBuilder$$anonfun$constructorsAndCompanion$3.apply(Reflector.scala:116)
  at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:245)
  at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:245)
  at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
  at scala.collection.mutable.WrappedArray.foreach(WrappedArray.scala:35)
  at scala.collection.TraversableLike$class.map(TraversableLike.scala:245)
  at scala.collection.AbstractTraversable.map(Traversable.scala:104)
  at org.json4s.reflect.Reflector$ClassDescriptorBuilder.constructorsAndCompanion(Reflector.scala:116)
  at org.json4s.reflect.Reflector$ClassDescriptorBuilder.result(Reflector.scala:156)
  at org.json4s.reflect.Reflector$.createDescriptor(Reflector.scala:50)
  at org.json4s.reflect.Reflector$$anonfun$describe$1.apply(Reflector.scala:44)
  at org.json4s.reflect.Reflector$$anonfun$describe$1.apply(Reflector.scala:44)
  at org.json4s.reflect.package$Memo.apply(package.scala:39)
  at org.json4s.reflect.Reflector$.describe(Reflector.scala:44)
  at org.json4s.Extraction$.decomposeObject$1(Extraction.scala:91)
  at org.json4s.Extraction$.internalDecomposeWithBuilder(Extraction.scala:180)
  at org.json4s.Extraction$.decomposeWithBuilder(Extraction.scala:67)
  at org.json4s.Extraction$.decompose(Extraction.scala:194)```

Reproduce the Scala 2.11.1 REPL example, comparing extraction of Test with parameterized Test2. Start with reflect/ScalaSigReader.scala:21 and reflect/Reflector.scala, where the stack trace reports constructor discovery. Done means parameterized case classes no longer fail with MappingException during Extraction.decompose, with regression coverage for the reported example.