Subclass exposes its internal supertype

[tieskedh]

class Public : PublicInterface<T> by InternalImpl<Int>()

I believe the answer to the problem itself is given here:

Why does Kotlin prohibit exposing restricted-visibility types?

The reason is this. Kotlin’s default visibility is public, and the goal is that you don’t have to pollute your internal and private classes with explicit visibility modifier. Declaring your class as private or internal should be enough to hide it from the outside world. Now, if this class happens to be open and you inherit from it a public class, then you accidentally leak details that were originally supposed to be private/internal to the outside world. This is error-prone (hard to notice) and …