import java.util.HashMap;
import java.util.Map;
import java.util.function.BiFunction;

public class MapMerge {
  public static void main(String[] args) {
    Map<String, String> map = new HashMap<>();
    map.put("k", "v");
    // :: error: (return)
    map.merge("k", "v", (a, b) -> null).toString();
  }

  void foo(Map<String, String> map) {
    // :: error: (return)
    merge(map, "k", "v", (a, b) -> null).toString();
  }

  <K, V> V merge(
      Map<K, V> map,
      K key,
      V value,
      BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
    return value;
  }
}