date_add/date_sub can return `mysql.Time`.

[winoros]

Bug Report

Although in MySQL's docs:

The return value depends on the arguments:

DATE if the date argument is a DATE value and your calculations involve only YEAR, MONTH, and DAY parts (that is, no time parts).

DATETIME if the first argument is a DATETIME (or TIMESTAMP) value, or if the first argument is a DATE and the unit value uses HOURS, MINUTES, or SECONDS.

String otherwise.

But in its actual implementation:

bool Item_date_add_interval::resolve_type(THD *) {
  maybe_null = true;

  /*
    The field type for the result of an Item_date function is defined as
    follows:

    - If first arg is a MYSQL_TYPE_DATETIME result is MYSQL_TYPE_DATETIME
    - If first arg is a MYSQL_TYPE_DATE and the interval type uses hours,
      minutes or seconds then type is MYSQL_TYPE_DATETIME.
    - Otherwise the result is MYSQL_TYPE_STRING
      (This is because you can't know if the string contains a DATE, MYSQL_TIME
    or DATETIME argument)
  */
  enum_field_types arg0_data_type = args[0]->data_type();
  uint8 interval_dec = 0;
  if (int_type == INTERVAL_MICROSECOND ||
      (int_type >= INTERVAL_DAY_MICROSECOND &&
       int_type <= INTERVAL_SECOND_MICROSECOND))
    interval_dec = DATETIME_MAX_DECIMALS;
  else if (int_type == INTERVAL_SECOND && args[1]->decimals > 0)
    interval_dec = MY_MIN(args[1]->decimals, DATETIME_MAX_DECIMALS);

  if (arg0_data_type == MYSQL_TYPE_DATETIME ||
      arg0_data_type == MYSQL_TYPE_TIMESTAMP) {
    uint8 dec = MY_MAX(args[0]->datetime_precision(), interval_dec);
    set_data_type_datetime(dec);
  } else if (arg0_data_type == MYSQL_TYPE_DATE) {
    if (int_type <= INTERVAL_DAY || int_type == INTERVAL_YEAR_MONTH)
      set_data_type_date();
    else
      set_data_type_datetime(interval_dec);
  } else if (arg0_data_type == MYSQL_TYPE_TIME) {
    uint8 dec = MY_MAX(args[0]->time_precision(), interval_dec);
    set_data_type_time(dec);
  } else {
    /* Behave as a usual string function when return type is VARCHAR. */
    set_data_type_char(MAX_DATETIME_FULL_WIDTH, default_charset());
  }
  if (value.alloc(max_length)) return true;

  return false;
}

The return type can also be Time, which is Duration in TiDB.

So currently if the arg[0] is Duration we'll get the result with the year, month and day field. Which is not a reasonable return value.
We need to cover this unexpected case.