Inheritance of AbstractAuthenticationBuilder in Kotlin #19805

Expected Behavior

AbstractAuthenticationBuilder class should be public for it to be inherited by a public class in Kotlin.

Current Behavior

The following code

open class Builder<B : Builder<B>>(token: MyAuthentication) : AbstractAuthenticationBuilder<B>(token) {
    ...
}

does not compile with error 'public' subclass exposes its 'protected (in AbstractAuthenticationToken)' supertype 'AbstractAuthenticationBuilder'..

Custom Builder class should in turn be public because it is exposed by public MyAuthentication.toBuilder() method.