projections are not allowed for immediate subtypes of a supertype

Question

I have an abstract class like the following:

abstract class AbstractSync<out C : Configuration<*>> : ISync {
    internal abstract val configuration: C
    ...
}

I want another abstract class that inherits from this:

abstract class CascadedSync : AbstractSync {}

The CascadedSync class should not modify the generic parameter C, it should, in fact, only implement a single method of ISync and nothing else.

I am trying out different syntaxes to achive this seemingly simple task. In Java, this would look just like this:

abstract class CascadedSync extends AbstractSync {    
}

Translating it to Kotlin with the help of IntelliJ produces this:

abstract class CascadedSync : AbstractSync<*>()

But at the same time gives the following error:

projections are not allowed for immediate subtypes of a supertype

What is the right syntax?

Answer 1

In Kotlin, unlike Java, type parameters are not implicitly copied from types to their subtypes and, even if you don't modify them, they should be redeclared in the subtype declaration:

abstract class CascadedSync<out C : Configuration<*>> : AbstractSync<C>() {
    // ...
}