Kotlin 错误：public function exposes its 'public*package*' return type argument

我是Kotlin 的新手，正在try 用这种语言写一些项目.

'public' function exposes its 'public/*package*/' return type argument FooSettings

I understand the problem is but I am not sure how to fix it in Kotlin since I am still trying get familiar with Kotlin. I can see that Kotlin is being smart and only trying to return of type that extends FooSettings. However the problem is FooSettings is package public only which means that I cannot access if in my Kotlin project.

我做了一些关于Kotlin泛型和使用in或out的研究，但是我无法解决这个问题.

在我的Kotlin项目中，我能做些什么来修复我看到的错误吗？

代码片段

This is sample of Java library class:个 注意，我没有办法更改库的实现.我必须使用这个图书馆，并将其扩展到Kotlin .

对我来说，java库是以这种方式编写的，并期望它被重写，这似乎很奇怪，但这是另一天的问题.

import java.util.Collections;
import java.util.List;

public abstract class ClassA {
    public List<FooBuilder<?>> getBuilder(Foo foo) {
        return Collections.emptyList();
    }
}

public class Foo {
}

public abstract class FooBuilder<U extends FooBuilder.FooSettings> {
    // implementation of Class
    abstract static class FooSettings {
        // implementation of Class
    }
}

Normally Java classes would override the method like such:

import java.util.List;

public class MyJavaClassA extends ClassA {
    @Override public List<FooBuilder<?>> getBuilder(final Foo foo) {
        // implementation
    }
}

But I am trying to write in Kotlin such that it looks like:提醒一下，这个Kotlin依赖于Java库，并且没有访问包公共类的权限.

class MyKotlinClassA : ClassA() {
    override fun getBuilder(foo: Foo): MutableList<FooBuilder<*>> {
        // implementation
    }
}

This causes error

'public' function exposes its 'public/*package*/' return type argument FooSettings