null unboxing is broken because the backend does not intercept the unbox calls

[smarter]

This code:

object Test {
  def main(args: Array[String]): Unit = {
    val d: Double = null.asInstanceOf[Double]
    println(d)
  }
}

Is transformed both in scalac and dotty into:

def main(args: String[]): Unit = {
  val d: Double = scala.Double.unbox(null)
  println(scala.Double.box(d))
}

But this is just an illusion: in scalac, the unbox call is actually replaced by BoxesRunTime.unboxToDouble(null). Dotty does not do this replacement and so the call fails with a NullPointerException.

(I still wonder why there even is an implementation in Double.scala since it's both unused and incorrect: https://github.com/scala/scala/blob/2.11.x/src/library/scala/Double.scala#L244)

[smarter]

Needed to get tests/pending/run/t5866.scala to run.