Unfriendly error: Unsafe instantation of type => List[(java.util.List[_] @UnsafeNonvariant)#java$util$List$$E]

[smarter]

While trying to compile dottydoc I ran across this error, here's a minimization:

import scala.collection.JavaConverters._

object Test {
  def test(x: Any): Unit = {
    x.asInstanceOf[java.util.List[_]].asScala.toList
  }
}

-- Error: jlist.scala ------------------------------------------------------
5 |    x.asInstanceOf[java.util.List[_]].asScala.toList
  |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |    unsafe instantiation of type => 
  |      scala.collection.immutable.List[
  |        java.util.List'[_] @UnsafeNonvariant#java$util$List$$E
  |      ]
  |    (?1.toList)where:    List  is a class in package immutable
  |              List' is a trait in package util

The error disappear if the code is rewritten as:

import scala.collection.JavaConverters._

object Test {
  def test(x: Any): Unit = {
    val y = x.asInstanceOf[java.util.List[_]].asScala
    y.toList
  }
}

@odersky Could you explain why this happens exactly? Is this something that we can fix? Otherwise, we should try to give a better error message to the user, this one does not help you fix the problem. Ideally, the compiler would tell you to do the rewriting above.