Unsoundness due to initialization-produced null

[sir-wabbit]

Similar to #4031. Derived from this gist, which works in Scala2.

object App{
 def coerce[U, V](u: U): V ={
 trait X{type R >: U }
 trait Y{type R = V }

 class T[A <: X](val a: A)(val value: a.R)

 object O{val x : Y & X = x }

 val a = new T[Y & X](O.x)(u)
 a.value
 }

 def main(args: Array[String]): Unit ={
 val x: Int = coerce[String, Int]("a")
 println(x + 1)
 }
}

/////////////////////////////////

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
	at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:101)
	at App$.main(HelloWorld.scala:15)
	at App.main(HelloWorld.scala)

Tested on the most recent master branch.

[Blaisorblade]

Ok, finally found time to read it. I’m almost asleep, but I’m guessing that O.x ends up being null, assuming that’s right, we know null introduces unsoundness in Scala and Java (all versions) and null-tracking is not yet implemented, though we plan to get to it. I’ll figure out how to sort this snippet among “pending” testcases for null-tracking.

[Blaisorblade]

Confirm that O.x is null.
So the issue is a duplicate of (the null-related part of) #50 (see #50 (comment) for the latest comment).