Fix linking issues from subgroups to epics in parent group

ee/app/services/epic_issues/create_service.rb

@@ -19,7 +19,11 @@ module EpicIssues
     def linkable_issues(issues)
       return [] unless can?(current_user, :admin_epic, issuable.group)
 
-      issues.select { |issue| issue.project.group == issuable.group }
+      issues.select { |issue| issuable_group_descendants.include?(issue.project.group) }
+    end
+
+    def issuable_group_descendants
+      @descendants ||= issuable.group.self_and_descendants
     end
   end
 end

spec/ee/spec/services/epic_issues/create_service_spec.rb

@@ -88,7 +88,17 @@ describe EpicIssues::CreateService do
             end
           end
 
-          context 'when an issue links is given' do
+          context 'when an issue link is given' do
+            subject { assign_issue([IssuesHelper.url_for_issue(issue.iid, issue.project)]) }
+
+            include_examples 'returns success'
+          end
+
+          context 'when a link of an issue in a subgroup is given', :nested_groups do
+            let(:subgroup) { create(:group, parent: group) }
+            let(:project2) { create(:project, group: subgroup) }
+            let(:issue) { create(:issue, project: project2) }
+
             subject { assign_issue([IssuesHelper.url_for_issue(issue.iid, issue.project)]) }
 
             include_examples 'returns success'