Scala TypeTag的奇怪行为未显示类型参数

Question

I have the following code snippet:

import scala.reflect.runtime.universe._

def paramInfo[N: TypeTag](x: N): Unit = {
  val targs = typeOf[N] match { case TypeRef(_, _, args) => args }
  println(s"type of $x has type arguments $targs")
}

case class Dummy(l: List[Int])

import scala.reflect.runtime.universe._
paramInfo: [N](x: N)(implicit evidence$1: reflect.runtime.universe.TypeTag[N])Unit
defined class Dummy

// Exiting paste mode, now interpreting.

type of Dummy(List(1, 2)) has type arguments List()

scala> paramInfo(List(1,2))
type of List(1, 2) has type arguments List(Int)

scala> paramInfo(Dummy(List(1,2)))
type of Dummy(List(1, 2)) has type arguments List()

What I do not understand is that I was expecting the call paramInfo(Dummy(List(1,2))) to actually print:

type of Dummy(List(1, 2)) has type arguments List(Dummy(List(Int)))

Did I get it wrong? Any reason?

EDIT: After comments from Dima, I created a generic Dummy and this time I was expecting the type arguments to beList(Dummy(List(Int))), but it is not? Why?

scala> case class Dummy[X](l: List[X])
defined class Dummy

scala> Dummy[Int](List(1,2))
res64: Dummy[Int] = Dummy(List(1, 2))

scala> paramInfo(res64)
type of Dummy(List(1, 2)) has type arguments List(Int)

Answer 1

So, args in TypeRef is a list of type parameters.

Dummy does not have type parameters, so you get a an empty List() back. List[Int] has one type parameter - Int - so, you get a list of a single element, which is Int .

I am not sure I understand why you were expecting to see what you said, so I can't help there ... why do you expect Dummy to be a parameter to itself???

BTW, you can write your match statement shorter:

 val TypeRef(_, _, targs) = typeOf[N]