assertThrows(NullPointerException.class, () -> copyOf((Collection<String>) c));
  }

  enum TestEnum {
    A,
    B,
    C,
    D
  }

  public void testCopyOf_collection_enumSet() {
    Collection<TestEnum> c = EnumSet.of(TestEnum.A, TestEnum.B, TestEnum.D);
    Set<TestEnum> set = copyOf(c);
    assertEquals(3, set.size());
    assertEquals(c, set);
  }