func (s *IssuesService) UpdateIssue(pid interface{}, issue int, opt *UpdateIssueOptions, options ...RequestOptionFunc) (*Issue, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/issues/%d", pathEscape(project), issue)
	req, err := s.client.NewRequest(http.MethodPut, u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	i := new(Issue)
	resp, err := s.client.Do(req, i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
UpdateIssue updates an existing project issue. This function is also used
to mark an issue as closed.

GitLab API docs: https://docs.gitlab.com/ce/api/issues.html#edit-issues
package gitlab
import (
	"bytes"
	"encoding/json"
	"fmt"
	"net/http"
	"net/url"
	"reflect"
	"strings"
	"time"
)
type IssuesService struct {
	client    *Client
	timeStats *timeStatsService
}
type IssueAuthor struct {
	ID        int    `json:"id"`
	State     string `json:"state"`
	WebURL    string `json:"web_url"`
	Name      string `json:"name"`
	AvatarURL string `json:"avatar_url"`
	Username  string `json:"username"`
}
type IssueAssignee struct {
	ID        int    `json:"id"`
	State     string `json:"state"`
	WebURL    string `json:"web_url"`
	Name      string `json:"name"`
	AvatarURL string `json:"avatar_url"`
	Username  string `json:"username"`
}
type IssueReferences struct {
	Short    string `json:"short"`
	Relative string `json:"relative"`
	Full     string `json:"full"`
}
type IssueCloser struct {
	ID        int    `json:"id"`
	State     string `json:"state"`
	WebURL    string `json:"web_url"`
	Name      string `json:"name"`
	AvatarURL string `json:"avatar_url"`
	Username  string `json:"username"`
}
type IssueLinks struct {
	Self       string `json:"self"`
	Notes      string `json:"notes"`
	AwardEmoji string `json:"award_emoji"`
	Project    string `json:"project"`
}
type Issue struct {
	ID                   int                    `json:"id"`
	IID                  int                    `json:"iid"`
	ExternalID           string                 `json:"external_id"`
	State                string                 `json:"state"`
	Description          string                 `json:"description"`
	Author               *IssueAuthor           `json:"author"`
	Milestone            *Milestone             `json:"milestone"`
	ProjectID            int                    `json:"project_id"`
	Assignees            []*IssueAssignee       `json:"assignees"`
	Assignee             *IssueAssignee         `json:"assignee"`
	UpdatedAt            *time.Time             `json:"updated_at"`
	ClosedAt             *time.Time             `json:"closed_at"`
	ClosedBy             *IssueCloser           `json:"closed_by"`
	Title                string                 `json:"title"`
	CreatedAt            *time.Time             `json:"created_at"`
	MovedToID            int                    `json:"moved_to_id"`
	Labels               Labels                 `json:"labels"`
	LabelDetails         []*LabelDetails        `json:"label_details"`
	Upvotes              int                    `json:"upvotes"`
	Downvotes            int                    `json:"downvotes"`
	DueDate              *ISOTime               `json:"due_date"`
	WebURL               string                 `json:"web_url"`
	References           *IssueReferences       `json:"references"`
	TimeStats            *TimeStats             `json:"time_stats"`
	Confidential         bool                   `json:"confidential"`
	Weight               int                    `json:"weight"`
	DiscussionLocked     bool                   `json:"discussion_locked"`
	IssueType            *string                `json:"issue_type,omitempty"`
	Subscribed           bool                   `json:"subscribed"`
	UserNotesCount       int                    `json:"user_notes_count"`
	Links                *IssueLinks            `json:"_links"`
	IssueLinkID          int                    `json:"issue_link_id"`
	MergeRequestCount    int                    `json:"merge_requests_count"`
	EpicIssueID          int                    `json:"epic_issue_id"`
	Epic                 *Epic                  `json:"epic"`
	TaskCompletionStatus *TasksCompletionStatus `json:"task_completion_status"`
}
func (i Issue) String() string {
	return Stringify(i)
}
func (i *Issue) UnmarshalJSON(data []byte) error {
	type alias Issue
	raw := make(map[string]interface{})
	err := json.Unmarshal(data, &raw)
	if err != nil {
		return err
	}
	if reflect.TypeOf(raw["id"]).Kind() == reflect.String {
		raw["external_id"] = raw["id"]
		delete(raw, "id")
	}
	labelDetails, ok := raw["labels"].([]interface{})
	if ok && len(labelDetails) > 0 {
		if _, ok := labelDetails[0].(map[string]interface{}); ok {
			labels := make([]interface{}, len(labelDetails))
			for i, details := range labelDetails {
				labels[i] = details.(map[string]interface{})["name"]
			}
			raw["labels"] = labels
			raw["label_details"] = labelDetails
		}
	}
	data, err = json.Marshal(raw)
	if err != nil {
		return err
	}
	return json.Unmarshal(data, (*alias)(i))
}
type Labels []string
func (l *Labels) MarshalJSON() ([]byte, error) {
	if *l == nil {
		return []byte(`null`), nil
	}
	return json.Marshal(strings.Join(*l, ","))
}
func (l *Labels) UnmarshalJSON(data []byte) error {
	type alias Labels
	if !bytes.HasPrefix(data, []byte("[")) {
		data = []byte(fmt.Sprintf("[%s]", string(data)))
	}
	return json.Unmarshal(data, (*alias)(l))
}
func (l *Labels) EncodeValues(key string, v *url.Values) error {
	v.Set(key, strings.Join(*l, ","))
	return nil
}
type LabelDetails struct {
	ID              int    `json:"id"`
	Name            string `json:"name"`
	Color           string `json:"color"`
	Description     string `json:"description"`
	DescriptionHTML string `json:"description_html"`
	TextColor       string `json:"text_color"`
}
type ListIssuesOptions struct {
	ListOptions
	State              *string    `url:"state,omitempty" json:"state,omitempty"`
	Labels             Labels     `url:"labels,comma,omitempty" json:"labels,omitempty"`
	NotLabels          Labels     `url:"not[labels],comma,omitempty" json:"not[labels],omitempty"`
	WithLabelDetails   *bool      `url:"with_labels_details,omitempty" json:"with_labels_details,omitempty"`
	Milestone          *string    `url:"milestone,omitempty" json:"milestone,omitempty"`
	NotMilestone       *string    `url:"not[milestone],omitempty" json:"not[milestone],omitempty"`
	Scope              *string    `url:"scope,omitempty" json:"scope,omitempty"`
	AuthorID           *int       `url:"author_id,omitempty" json:"author_id,omitempty"`
	NotAuthorID        []int      `url:"not[author_id],omitempty" json:"not[author_id],omitempty"`
	AssigneeID         *int       `url:"assignee_id,omitempty" json:"assignee_id,omitempty"`
	NotAssigneeID      []int      `url:"not[assignee_id],omitempty" json:"not[assignee_id],omitempty"`
	AssigneeUsername   *string    `url:"assignee_username,omitempty" json:"assignee_username,omitempty"`
	MyReactionEmoji    *string    `url:"my_reaction_emoji,omitempty" json:"my_reaction_emoji,omitempty"`
	NotMyReactionEmoji []string   `url:"not[my_reaction_emoji],omitempty" json:"not[my_reaction_emoji],omitempty"`
	IIDs               []int      `url:"iids[],omitempty" json:"iids,omitempty"`
	In                 *string    `url:"in,omitempty" json:"in,omitempty"`
	OrderBy            *string    `url:"order_by,omitempty" json:"order_by,omitempty"`
	Sort               *string    `url:"sort,omitempty" json:"sort,omitempty"`
	Search             *string    `url:"search,omitempty" json:"search,omitempty"`
	CreatedAfter       *time.Time `url:"created_after,omitempty" json:"created_after,omitempty"`
	CreatedBefore      *time.Time `url:"created_before,omitempty" json:"created_before,omitempty"`
	UpdatedAfter       *time.Time `url:"updated_after,omitempty" json:"updated_after,omitempty"`
	UpdatedBefore      *time.Time `url:"updated_before,omitempty" json:"updated_before,omitempty"`
	Confidential       *bool      `url:"confidential,omitempty" json:"confidential,omitempty"`
	IssueType          *string    `url:"issue_type,omitempty" json:"issue_type,omitempty"`
}
func (s *IssuesService) ListIssues(opt *ListIssuesOptions, options ...RequestOptionFunc) ([]*Issue, *Response, error) {
	req, err := s.client.NewRequest(http.MethodGet, "issues", opt, options)
	if err != nil {
		return nil, nil, err
	}
	var i []*Issue
	resp, err := s.client.Do(req, &i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
type ListGroupIssuesOptions struct {
	ListOptions
	State              *string    `url:"state,omitempty" json:"state,omitempty"`
	Labels             Labels     `url:"labels,comma,omitempty" json:"labels,omitempty"`
	NotLabels          Labels     `url:"not[labels],comma,omitempty" json:"not[labels],omitempty"`
	WithLabelDetails   *bool      `url:"with_labels_details,omitempty" json:"with_labels_details,omitempty"`
	IIDs               []int      `url:"iids[],omitempty" json:"iids,omitempty"`
	Milestone          *string    `url:"milestone,omitempty" json:"milestone,omitempty"`
	NotMilestone       *string    `url:"not[milestone],omitempty" json:"not[milestone],omitempty"`
	Scope              *string    `url:"scope,omitempty" json:"scope,omitempty"`
	AuthorID           *int       `url:"author_id,omitempty" json:"author_id,omitempty"`
	NotAuthorID        []int      `url:"not[author_id],omitempty" json:"not[author_id],omitempty"`
	AuthorUsername     *string    `url:"author_username,omitempty" json:"author_username,omitempty"`
	AssigneeID         *int       `url:"assignee_id,omitempty" json:"assignee_id,omitempty"`
	NotAssigneeID      []int      `url:"not[assignee_id],omitempty" json:"not[assignee_id],omitempty"`
	AssigneeUsername   *string    `url:"assignee_username,omitempty" json:"assignee_username,omitempty"`
	MyReactionEmoji    *string    `url:"my_reaction_emoji,omitempty" json:"my_reaction_emoji,omitempty"`
	NotMyReactionEmoji []string   `url:"not[my_reaction_emoji],omitempty" json:"not[my_reaction_emoji],omitempty"`
	OrderBy            *string    `url:"order_by,omitempty" json:"order_by,omitempty"`
	Sort               *string    `url:"sort,omitempty" json:"sort,omitempty"`
	Search             *string    `url:"search,omitempty" json:"search,omitempty"`
	In                 *string    `url:"in,omitempty" json:"in,omitempty"`
	CreatedAfter       *time.Time `url:"created_after,omitempty" json:"created_after,omitempty"`
	CreatedBefore      *time.Time `url:"created_before,omitempty" json:"created_before,omitempty"`
	UpdatedAfter       *time.Time `url:"updated_after,omitempty" json:"updated_after,omitempty"`
	UpdatedBefore      *time.Time `url:"updated_before,omitempty" json:"updated_before,omitempty"`
	IssueType          *string    `url:"issue_type,omitempty" json:"issue_type,omitempty"`
}
func (s *IssuesService) ListGroupIssues(pid interface{}, opt *ListGroupIssuesOptions, options ...RequestOptionFunc) ([]*Issue, *Response, error) {
	group, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("groups/%s/issues", pathEscape(group))
	req, err := s.client.NewRequest(http.MethodGet, u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	var i []*Issue
	resp, err := s.client.Do(req, &i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
type ListProjectIssuesOptions struct {
	ListOptions
	IIDs               []int      `url:"iids[],omitempty" json:"iids,omitempty"`
	State              *string    `url:"state,omitempty" json:"state,omitempty"`
	Labels             Labels     `url:"labels,comma,omitempty" json:"labels,omitempty"`
	NotLabels          Labels     `url:"not[labels],comma,omitempty" json:"not[labels],omitempty"`
	WithLabelDetails   *bool      `url:"with_labels_details,omitempty" json:"with_labels_details,omitempty"`
	Milestone          *string    `url:"milestone,omitempty" json:"milestone,omitempty"`
	NotMilestone       []string   `url:"not[milestone],omitempty" json:"not[milestone],omitempty"`
	Scope              *string    `url:"scope,omitempty" json:"scope,omitempty"`
	AuthorID           *int       `url:"author_id,omitempty" json:"author_id,omitempty"`
	NotAuthorID        []int      `url:"not[author_id],omitempty" json:"not[author_id],omitempty"`
	AssigneeID         *int       `url:"assignee_id,omitempty" json:"assignee_id,omitempty"`
	NotAssigneeID      []int      `url:"not[assignee_id],omitempty" json:"not[assignee_id],omitempty"`
	AssigneeUsername   *string    `url:"assignee_username,omitempty" json:"assignee_username,omitempty"`
	MyReactionEmoji    *string    `url:"my_reaction_emoji,omitempty" json:"my_reaction_emoji,omitempty"`
	NotMyReactionEmoji []string   `url:"not[my_reaction_emoji],omitempty" json:"not[my_reaction_emoji],omitempty"`
	OrderBy            *string    `url:"order_by,omitempty" json:"order_by,omitempty"`
	Sort               *string    `url:"sort,omitempty" json:"sort,omitempty"`
	Search             *string    `url:"search,omitempty" json:"search,omitempty"`
	In                 *string    `url:"in,omitempty" json:"in,omitempty"`
	CreatedAfter       *time.Time `url:"created_after,omitempty" json:"created_after,omitempty"`
	CreatedBefore      *time.Time `url:"created_before,omitempty" json:"created_before,omitempty"`
	DueDate            *string    `url:"due_date,omitempty" json:"due_date,omitempty"`
	UpdatedAfter       *time.Time `url:"updated_after,omitempty" json:"updated_after,omitempty"`
	UpdatedBefore      *time.Time `url:"updated_before,omitempty" json:"updated_before,omitempty"`
	Confidential       *bool      `url:"confidential,omitempty" json:"confidential,omitempty"`
	IssueType          *string    `url:"issue_type,omitempty" json:"issue_type,omitempty"`
}
func (s *IssuesService) ListProjectIssues(pid interface{}, opt *ListProjectIssuesOptions, options ...RequestOptionFunc) ([]*Issue, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/issues", pathEscape(project))
	req, err := s.client.NewRequest(http.MethodGet, u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	var i []*Issue
	resp, err := s.client.Do(req, &i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
func (s *IssuesService) GetIssue(pid interface{}, issue int, options ...RequestOptionFunc) (*Issue, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/issues/%d", pathEscape(project), issue)
	req, err := s.client.NewRequest(http.MethodGet, u, nil, options)
	if err != nil {
		return nil, nil, err
	}
	i := new(Issue)
	resp, err := s.client.Do(req, i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
type CreateIssueOptions struct {
	IID                                *int       `url:"iid,omitempty" json:"iid,omitempty"`
	Title                              *string    `url:"title,omitempty" json:"title,omitempty"`
	Description                        *string    `url:"description,omitempty" json:"description,omitempty"`
	Confidential                       *bool      `url:"confidential,omitempty" json:"confidential,omitempty"`
	AssigneeIDs                        []int      `url:"assignee_ids,omitempty" json:"assignee_ids,omitempty"`
	MilestoneID                        *int       `url:"milestone_id,omitempty" json:"milestone_id,omitempty"`
	Labels                             Labels     `url:"labels,comma,omitempty" json:"labels,omitempty"`
	CreatedAt                          *time.Time `url:"created_at,omitempty" json:"created_at,omitempty"`
	DueDate                            *ISOTime   `url:"due_date,omitempty" json:"due_date,omitempty"`
	MergeRequestToResolveDiscussionsOf *int       `url:"merge_request_to_resolve_discussions_of,omitempty" json:"merge_request_to_resolve_discussions_of,omitempty"`
	DiscussionToResolve                *string    `url:"discussion_to_resolve,omitempty" json:"discussion_to_resolve,omitempty"`
	Weight                             *int       `url:"weight,omitempty" json:"weight,omitempty"`
	IssueType                          *string    `url:"issue_type,omitempty" json:"issue_type,omitempty"`
}
func (s *IssuesService) CreateIssue(pid interface{}, opt *CreateIssueOptions, options ...RequestOptionFunc) (*Issue, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/issues", pathEscape(project))
	req, err := s.client.NewRequest(http.MethodPost, u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	i := new(Issue)
	resp, err := s.client.Do(req, i)
	if err != nil {
		return nil, resp, err
	}
	return i, resp, err
}
type UpdateIssueOptions struct {
	Title            *string    `url:"title,omitempty" json:"title,omitempty"`
	Description      *string    `url:"description,omitempty" json:"description,omitempty"`
	Confidential     *bool      `url:"confidential,omitempty" json:"confidential,omitempty"`
	AssigneeIDs      []int      `url:"assignee_ids,omitempty" json:"assignee_ids,omitempty"`
	MilestoneID      *int       `url:"milestone_id,omitempty" json:"milestone_id,omitempty"`
	Labels           Labels     `url:"labels,comma,omitempty" json:"labels,omitempty"`
	AddLabels        Labels     `url:"add_labels,comma,omitempty" json:"add_labels,omitempty"`
	RemoveLabels     Labels     `url:"remove_labels,comma,omitempty" json:"remove_labels,omitempty"`
	StateEvent       *string    `url:"state_event,omitempty" json:"state_event,omitempty"`
	UpdatedAt        *time.Time `url:"updated_at,omitempty" json:"updated_at,omitempty"`
	DueDate          *ISOTime   `url:"due_date,omitempty" json:"due_date,omitempty"`
	Weight           *int       `url:"weight,omitempty" json:"weight,omitempty"`
	DiscussionLocked *bool      `url:"discussion_locked,omitempty" json:"discussion_locked,omitempty"`
	IssueType        *string    `url:"issue_type,omitempty" json:"issue_type,omitempty"`
}