// Test case for Issue 1838:
// https://github.com/typetools/checker-framework/issues/1838

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

public class Issue1838 {
  public static void main(String[] args) {
    f();
  }

  public static void f() {
    List<@Nullable Object> list = new ArrayList<>();
    list.add(null);
    List<List<@Nullable Object>> listList = new ArrayList<List<@Nullable Object>>();
    listList.add(list);
    // :: error: (argument)
    processElements(listList);
  }

  private static void processElements(List<? extends List<Object>> listList) {
    for (List<Object> list : listList) {
      for (Object element : list) {
        element.toString();
      }
    }
  }
}