Unsoundness due to missing variance checks of private members

[joroKr21]

minimized code

class A
class B extends A
class C extends A

object Foo {
  private[this] class Bar[+T](var x: T)
  def foo: B = {
    val barB: Bar[B] = new Bar(new B)
    val barA: Bar[A] = barB
    barA.x = new C
    barB.x
  }
}

Foo.foo

result

java.lang.ClassCastException

expectation

Variance error is reported.

variations

• private is also broken in Dotty
• private[this] is also broken in Scalac
• private is detected in Scalac