import java.util.ArrayList;
import java.util.List;
import org.checkerframework.checker.nullness.qual.*;

public class Issue1949 {
    public interface Base<R> {}

    public interface Child<R> extends Base<@Nullable R> {}

    public abstract static class BaseClass<R> implements Child<R> {
        abstract List<Child<R>> foo();
    }

    public static class ChildClass extends BaseClass<String> {

        @Override
        public List<Child<String>> foo() {
            return new ArrayList<>();
        }
    }
}