@Test
public void optionalField_withDefaultValue_detectsChange() {

  Campaign.Builder builder = Campaign.newBuilder();
  builder.getManualCpcBuilder().setEnhancedCpcEnabled(false);
  Campaign campaign = builder.build();
  assertEquals(
      ImmutableList.of("manual_cpc.enhanced_cpc_enabled"),
      FieldMasks.allSetFieldsOf(campaign).getPathsList());
}