IArray.toArray is potentially unsafe (ArrayStoreException, ClassCastException)

[xuwei-k]

Compiler version

3.0.0

Minimized code

scala> val a: IArray[Some[Int]] = IArray(Some(1))
val a: IArray[Some[Int]] = Array(Some(1))

scala> val b: IArray[Option[Int]] = a
val b: IArray[Option[Int]] = Array(Some(1))

scala> val c = b.toArray
val c: Array[Option[Int]] = Array(Some(1))

scala> c.getClass                                                                                                                 
val res0: Class[? <: Array[Option[Int]]] = class [Lscala.Some;

scala> c(0) = None
java.lang.ArrayStoreException: scala.None$
  ... 28 elided

Output

Expectation

b.toArray should return Array[Option[?]] instead of Array[Some[?]] ? 🤔
Is this by design or bug 🤔

https://github.com/lampepfl/dotty/blob/release-3.0.0/library/src/scala/IArray.scala#L252-L254

[xuwei-k]

Another example

scala> val a: IArray[Int] = IArray(2)
val a: IArray[Int] = Array(2)

scala> val b: IArray[Any] = a
val b: IArray[Any] = Array(2)

scala> b.toArray                                                                                                                  
java.lang.ClassCastException: [I cannot be cast to [Ljava.lang.Object;

[smarter]

We need to return Array[? <: T], thankfully that should be binary-compatible since they both erase to Object here.

[xuwei-k]

#12598