Improve errore when setting confidential epics

- add support for multiple messages on backend
- fix handling errors on frontend

ee/app/assets/javascripts/vue_shared/components/sidebar/epics_select/store/actions.js

@@ -126,6 +126,11 @@ export const assignIssueToEpic = ({ state, dispatch }, epic) => {
         epic,
       });
     })
+    .catch((error) => {
+      // Handle specific format "#ID cannot be added: reason"
+      const message = error.response.data.message.split(':')[1].trim();
+      dispatch('receiveIssueUpdateFailure', message);
+    })
     .catch(() => {
       // Shows flash error for Epic change failure
       dispatch(

ee/app/models/ee/issue.rb

@@ -276,8 +276,16 @@ module EE
       return unless epic
 
       if !confidential? && epic.confidential?
-        errors.add :issue, _('Cannot set confidential epic for a non-confidential issue')
+        errors.add :issue, confidentiality_error
       end
     end
+
+    def confidentiality_error
+      if changed_attribute_names_to_save.include?('confidential')
+        return _('this issue cannot be made public since it belongs to a confidential epic')
+      end
+
+      _('this issue cannot be assigned to a confidential epic since it is public')
+    end
   end
 end

ee/changelogs/unreleased/238884-epic-error-messages.yml

+---
+title: Add a meaningful error message for assigning confidential epics to issues
+merge_request: 53078
+author:
+type: added

ee/spec/models/issue_spec.rb

@@ -280,22 +280,70 @@ RSpec.describe Issue do
     end
 
     describe 'confidential' do
-      subject { build(:issue, :confidential) }
+      let_it_be(:epic) { create(:epic, :confidential) }
 
-      it 'is valid when changing to non-confidential and is associated with non-confidential epic' do
-        subject.epic = build(:epic)
+      context 'when assigning an epic to a new issue' do
+        let(:issue) { build(:issue, confidential: confidential) }
 
-        subject.confidential = false
+        context 'when an issue is not confidential' do
+          let(:confidential) { false }
 
-        expect(subject).to be_valid
+          it 'is not valid' do
+            issue.epic = epic
+
+            expect(issue).not_to be_valid
+            expect(issue.errors.messages[:issue]).to include(/this issue cannot be assigned to a confidential epic since it is public/)
+          end
+        end
+
+        context 'when an issue is confidential' do
+          let(:confidential) { true }
+
+          it 'is valid' do
+            issue.epic = epic
+
+            expect(issue).to be_valid
+          end
+        end
       end
 
-      it 'is not valid when changing to non-confidential and is associated with confidential epic' do
-        subject.epic = build(:epic, :confidential)
+      context 'when updating an existing issue' do
+        let(:confidential) { true }
+        let(:issue) { create(:issue, confidential: confidential) }
 
-        subject.confidential = false
+        context 'when an issue is assigned to the confidential epic' do
+          before do
+            issue.update!(epic: epic)
+          end
 
-        expect(subject).not_to be_valid
+          context 'when changing issue to public' do
+            it 'is not valid' do
+              issue.confidential = false
+
+              expect(issue).not_to be_valid
+              expect(issue.errors.messages[:issue]).to include(/this issue cannot be made public since it belongs to a confidential epic/)
+            end
+          end
+        end
+
+        context 'when assigining a confidential issue' do
+          it 'is valid' do
+            issue.epic = epic
+
+            expect(issue).to be_valid
+          end
+        end
+
+        context 'when assigining a public issue' do
+          let(:confidential) { false }
+
+          it 'is not valid' do
+            issue.epic = epic
+
+            expect(issue).not_to be_valid
+            expect(issue.errors.messages[:issue]).to include(/this issue cannot be assigned to a confidential epic since it is public/)
+          end
+        end
       end
     end
   end

locale/gitlab.pot

@@ -35292,6 +35292,12 @@ msgstr ""
 msgid "this document"
 msgstr ""
 
+msgid "this issue cannot be assigned to a confidential epic since it is public"
+msgstr ""
+
+msgid "this issue cannot be made public since it belongs to a confidential epic"
+msgstr ""
+
 msgid "time summary"
 msgstr ""