zero/int.go
func (i *Int) UnmarshalJSON(data []byte) error {
	var err error
	var v interface{}
	if err = json.Unmarshal(data, &v); err != nil {
		return err
	}
	switch x := v.(type) {
	case float64:
		// Unmarshal again, directly to int64, to avoid intermediate float64
		err = json.Unmarshal(data, &i.Int64)
	case string:
		str := string(x)
		if len(str) == 0 {
			i.Valid = false
			return nil
		}
		i.Int64, err = strconv.ParseInt(str, 10, 64)
	case map[string]interface{}:
		err = json.Unmarshal(data, &i.NullInt64)
	case nil:
		i.Valid = false
		return nil
	default:
		err = fmt.Errorf("json: cannot unmarshal %v into Go value of type zero.Int", reflect.TypeOf(v).Name())
	}
	i.Valid = (err == nil) && (i.Int64 != 0)
	return err
}
func (i *Int) UnmarshalJSON(data []byte) error {
	var err error
	var v interface{}
	if err = json.Unmarshal(data, &v); err != nil {
		return err
	}
	switch x := v.(type) {
	case float64:
		// Unmarshal again, directly to int64, to avoid intermediate float64
		err = json.Unmarshal(data, &i.Int64)
	case string:
		str := string(x)
		if len(str) == 0 {
			i.Valid = false
			return nil
		}
		i.Int64, err = strconv.ParseInt(str, 10, 64)
	case map[string]interface{}:
		err = json.Unmarshal(data, &i.NullInt64)
	case nil:
		i.Valid = false
		return nil
	default:
		err = fmt.Errorf("json: cannot unmarshal %v into Go value of type zero.Int", reflect.TypeOf(v).Name())
	}
	i.Valid = (err == nil) && (i.Int64 != 0)
	return err
}
// UnmarshalJSON implements json.Unmarshaler.
// It supports number and null input.
// 0 will be considered a null Int.
// It also supports unmarshalling a sql.NullInt64.