code: func AddInt64(a int64, b int64) (int64, error) {
  	if (a > 0 && b > 0 && math.MaxInt64-a < b) ||
  		(a < 0 && b < 0 && math.MinInt64-a > b) {
  		return 0, ErrOverflow.GenWithStackByArgs("BIGINT", fmt.Sprintf("(%d, %d)", a, b))
  	}
  
  	return a + b, nil
  }
docs: // AddInt64 adds int64 a and b if no overflow, otherwise returns error.