Resolve "Milestones should not be set on epics (issue promotion)"

app/models/concerns/issuable.rb

@@ -427,6 +427,13 @@ module Issuable
   def wipless_title_changed(old_title)
     old_title != title
   end
+
+  ##
+  # Overridden on EE module
+  #
+  def supports_milestone?
+    respond_to?(:milestone_id)
+  end
 end
 
 Issuable.prepend(EE::Issuable) # rubocop: disable Cop/InjectEnterpriseEditionModule

app/services/issuable/clone/attributes_rewriter.rb

@@ -17,6 +17,8 @@ module Issuable
       private
 
       def cloneable_milestone
+        return unless new_entity.supports_milestone?
+
         title = original_entity.milestone&.title
         return unless title
 

ee/app/models/concerns/ee/issuable.rb

@@ -25,6 +25,11 @@ module EE
       super
     end
 
+    override :supports_milestone?
+    def supports_milestone?
+      super && !is_a?(Epic)
+    end
+
     def supports_epic?
       is_a?(Issue) && project.group
     end

ee/changelogs/unreleased/10270-milestones-should-not-be-set-on-epics-issue-promotion.yml

+---
+title: Do not include milestone attribute when promoting issue to epic
+merge_request: 14532
+author:
+type: fixed

ee/spec/models/concerns/ee/issuable_spec.rb

@@ -37,4 +37,17 @@ describe EE::Issuable do
       end
     end
   end
+
+  describe '#supports_milestone?' do
+    let(:group)   { create(:group) }
+    let(:project) { create(:project, group: group) }
+
+    context "for epics" do
+      let(:epic) { build(:epic) }
+
+      it 'returns false' do
+        expect(epic.supports_milestone?).to be_falsy
+      end
+    end
+  end
 end

ee/spec/services/ee/issuable/clone/attributes_rewriter_spec.rb

@@ -9,33 +9,35 @@ describe Issuable::Clone::AttributesRewriter do
   let(:original_issue) { create(:issue, project: project) }
 
   context 'when a new object is a group entity' do
-    let(:new_epic) { create(:epic, group: group) }
+    context 'when entity is an epic' do
+      let(:new_epic) { create(:epic, group: group) }
 
-    subject { described_class.new(user, original_issue, new_epic) }
+      subject { described_class.new(user, original_issue, new_epic) }
 
-    context 'setting labels' do
-      let!(:project_label1) { create(:label, title: 'label1', project: project) }
-      let!(:project_label2) { create(:label, title: 'label2', project: project) }
-      let!(:group_label1) { create(:group_label, title: 'group_label', group: group) }
-      let!(:group_label2) { create(:group_label, title: 'label2', group: group) }
+      context 'setting labels' do
+        let(:project_label1) { create(:label, title: 'label1', project: project) }
+        let!(:project_label2) { create(:label, title: 'label2', project: project) }
+        let(:group_label1) { create(:group_label, title: 'group_label', group: group) }
+        let!(:group_label2) { create(:group_label, title: 'label2', group: group) }
 
-      it 'keeps group labels and merges project labels where possible' do
-        original_issue.update(labels: [project_label1, project_label2, group_label1])
+        it 'keeps group labels and merges project labels where possible' do
+          original_issue.update(labels: [project_label1, project_label2, group_label1])
 
-        subject.execute
+          subject.execute
 
-        expect(new_epic.reload.labels).to match_array([group_label1, group_label2])
+          expect(new_epic.reload.labels).to match_array([group_label1, group_label2])
+        end
       end
-    end
 
-    context 'setting milestones' do
-      it 'sets milestone to nil when old issue milestone is not a group milestone' do
-        milestone = create(:milestone, title: 'milestone', project: project)
-        original_issue.update(milestone: milestone)
+      context 'setting milestones' do
+        it 'sets milestone attribute as nil' do
+          milestone = create(:milestone, title: 'milestone', group: group)
+          original_issue.update(milestone: milestone)
 
-        subject.execute
+          expect(new_epic).to receive(:update).with(labels: [], milestone: nil)
 
-        expect(new_epic.reload.milestone).to be_nil
+          subject.execute
+        end
       end
     end
   end

spec/models/concerns/issuable_spec.rb

@@ -774,4 +774,25 @@ describe Issuable do
       end
     end
   end
+
+  describe '#supports_milestone?' do
+    let(:group)   { create(:group) }
+    let(:project) { create(:project, group: group) }
+
+    context "for issues" do
+      let(:issue) { build(:issue, project: project) }
+
+      it 'returns true' do
+        expect(issue.supports_milestone?).to be_truthy
+      end
+    end
+
+    context "for merge requests" do
+      let(:merge_request) { build(:merge_request, target_project: project, source_project: project) }
+
+      it 'returns true' do
+        expect(merge_request.supports_milestone?).to be_truthy
+      end
+    end
+  end
 end