"java.lang.ClassCastException" due to compiler doesn't generate error "Covariant type T occurs in contravariant position in type T of value element" for private members"

[unkarjedy]

Compiler version

Scala 3.3.1

Minimized code

abstract class Animal { def name: String }
case class Dog(name: String) extends Animal
case class Cat(name: String) extends Animal

class Wrapper[+T](private var element: T)

object Wrapper {
  def main(args: Array[String]): Unit = {
    val dog: Wrapper[Dog] = new Wrapper(Dog("Rex"))
    val animal: Wrapper[Animal] = dog
    animal.element = Cat("Kitty")
    println(dog.element.name)
  }
}

Output

The code compiles, but when you run the program it crashes with error

Exception in thread "main" java.lang.ClassCastException: class Cat cannot be cast to class Dog (Cat and Dog are in unnamed module of loader 'app')
	at Wrapper$.main(Animal.scala:12)
	at Wrapper.main(Animal.scala)

Expectation

The code compiles in Scala 3 but didn't compile in 2.13.
It should produce error
Covariant type T occurs in contravariant position in type T of value element

The compiler shouldn't produce the error only when the visibility is private[this].
It looks like there is a typo somewhere in the compiler which checks for private, not private[this]

---

Related issue in The Scala Plugin, which actually shows the error:
https://youtrack.jetbrains.com/issue/SCL-21676

[sjrd]

Uh oh. That's a soundness issue. Thanks for the report.

What's worrying about it is that it casts doubt on the idea that private[this] can be inferred as an internal optimization of private. This issue highlights the fact that it holds dedicated semantics.

[sjrd]

This was fixed in #18693