Why does TypeTag not work for return types?

Question

It seems that TypeTags only work for type parameters that are used in the parameters of the called method, and not the return type:

scala> :paste
// Entering paste mode (ctrl-D to finish)

import scala.reflect.runtime.universe._

object Test {
  def withParam[T: TypeTag](v: T): T = {
    println(typeOf[T])
    0.asInstanceOf[T]
  }

  def justReturn[T: TypeTag](): T = {
    println(typeOf[T])
    0.asInstanceOf[T]
  }
}

// Exiting paste mode, now interpreting.

import scala.reflect.runtime.universe._
defined module Test


scala> val i: Int = Test.withParam(17)
Int
i: Int = 0

scala> val j: Int = Test.justReturn()
Nothing
j: Int = 0

This is consistent with the behaviour of Manifest in 2.9, but is there any reason it can't be done, and is there any other way to achieve this effect?

Answer 1

The type system starts with the most restrictive type (i.e. Nothing, of which there can be no instances; if there were it would be a godlike value able to stand in for anything and do anything). The type is then widened as needed, but since the return is in contravariant position, there's never a reason to widen. If you actually could return a Nothing, you'd be set in all situations.

You then subvert the type system by telling it that 0 is an instance of Nothing. It's completely false, of course, but the compiler dutifully believes you, and you rescue the situation by assigning it to an Int, which is what it really was all along. (It will also happily try to assign it to a String and you'll then get an exception at runtime because at that point it is nonsensical.)

In theory this could be done differently, but this is a pretty basic part of the type inference algorithm.