Allow promoting issue to epic in custom groups

When promoting an issue to an epic we should be able to specify in which
group the new epic is supposed to be created.

ee/app/services/epics/issue_promote_service.rb

@@ -4,9 +4,9 @@ module Epics
   class IssuePromoteService < ::Issuable::Clone::BaseService
     PromoteError = Class.new(StandardError)
 
-    def execute(issue)
+    def execute(issue, epic_group = nil)
       @issue = issue
-      @parent_group = issue.project.group
+      @parent_group = epic_group || issue.project.group
 
       validate_promotion!
 

ee/spec/services/epics/issue_promote_service_spec.rb

 # frozen_string_literal: true
 require 'spec_helper'
 
-RSpec.describe Epics::IssuePromoteService do
+RSpec.describe Epics::IssuePromoteService, :aggregate_failures do
   let(:user) { create(:user) }
   let(:group) { create(:group) }
   let(:project) { create(:project, group: group) }
@@ -41,8 +41,11 @@ RSpec.describe Epics::IssuePromoteService do
       end
 
       context 'when a user can promote the issue' do
+        let(:new_group) { create(:group) }
+
         before do
           group.add_developer(user)
+          new_group.add_developer(user)
         end
 
         context 'when an issue does not belong to a group' do
@@ -118,6 +121,19 @@ RSpec.describe Epics::IssuePromoteService do
           end
         end
 
+        context 'when promoting issue to a different group' do
+          it 'creates a new epic with correct attributes' do
+            epic = subject.execute(issue, new_group)
+
+            expect(issue.reload.promoted_to_epic_id).to eq(epic.id)
+            expect(epic.title).to eq(issue.title)
+            expect(epic.description).to eq(issue.description)
+            expect(epic.author).to eq(user)
+            expect(epic.group).to eq(new_group)
+            expect(epic.parent).to be_nil
+          end
+        end
+
         context 'when an issue belongs to an epic' do
           let(:parent_epic) { create(:epic, group: group) }
           let!(:epic_issue) { create(:epic_issue, epic: parent_epic, issue: issue) }
@@ -131,6 +147,35 @@ RSpec.describe Epics::IssuePromoteService do
             expect(epic.group).to eq(group)
             expect(epic.parent).to eq(parent_epic)
           end
+
+          context 'when promoting issue to a different group' do
+            it 'creates a new epic with correct attributes' do
+              expect { subject.execute(issue, new_group) }
+                .to raise_error(StandardError, 'Validation failed: Parent This epic cannot be added. An epic must belong to the same group or subgroup as its parent epic.')
+
+              expect(issue.reload.state).to eq("opened")
+              expect(issue.reload.promoted_to_epic_id).to be_nil
+            end
+          end
+
+          context 'when promoting issue to a different group in the hierarchy' do
+            let(:new_group) { create(:group, parent: group) }
+
+            before do
+              new_group.add_developer(user)
+            end
+
+            it 'creates a new epic with correct attributes' do
+              epic = subject.execute(issue, new_group)
+
+              expect(issue.reload.promoted_to_epic_id).to eq(epic.id)
+              expect(epic.title).to eq(issue.title)
+              expect(epic.description).to eq(issue.description)
+              expect(epic.author).to eq(user)
+              expect(epic.group).to eq(new_group)
+              expect(epic.parent).to eq(parent_epic)
+            end
+          end
         end
 
         context 'when issue was already promoted' do