Include subepics in IssuesFinder if requested

ee/app/finders/ee/issues_finder.rb

@@ -87,13 +87,21 @@ module EE
       params[:epic_id].to_s.downcase == ::IssuesFinder::FILTER_NONE
     end
 
+    def epics
+      if params[:include_subepics]
+        ::Gitlab::ObjectHierarchy.new(::Epic.for_ids(params[:epic_id])).base_and_descendants.select(:id)
+      else
+        params[:epic_id]
+      end
+    end
+
     def by_epic(items)
       return items unless by_epic?
 
       if filter_by_no_epic?
         items.no_epic
       else
-        items.in_epics(params[:epic_id])
+        items.in_epics(epics)
       end
     end
   end

ee/spec/finders/issues_finder_spec.rb

@@ -97,8 +97,20 @@ describe IssuesFinder do
         context 'filter issues by epic' do
           let(:params) { { epic_id: epic_1.id } }
 
-          it 'returns all issues in the epic, subepic issues excluded' do
-            expect(issues).to contain_exactly(issue_1)
+          context 'when include_subepics param is not included' do
+            it 'returns all issues in the epic, subepic issues excluded' do
+              expect(issues).to contain_exactly(issue_1)
+            end
+          end
+
+          context 'when include_subepics param is set to true' do
+            before do
+              params[:include_subepics] = true
+            end
+
+            it 'returns all issues in the epic including subepic issues' do
+              expect(issues).to contain_exactly(issue_1, issue_subepic)
+            end
           end
         end
       end