[35506] Avoid notifications for assigned/responsible placeholders

Right now, placeholder users cannot be watchers nor authors nor meeting
participants, so the only notifications that can happen are work package
assignees or responsibles.

https://community.openproject.com/wp/35506

app/models/work_package.rb

@@ -683,9 +683,10 @@ class WorkPackage < ApplicationRecord
     related = [author]
 
     [responsible, assigned_to].each do |user|
-      if user.is_a?(Group)
+      case user
+      when Group
         related += user.users
-      else
+      when User
         related << user
       end
     end

spec/features/work_packages/tabs/watcher_tab_spec.rb

@@ -136,4 +136,23 @@ describe 'Watcher tab', js: true, selenium: true do
       expect(page).to have_selector('.wp-tabs-count',  text: 1)
     end
   end
+
+  context 'with a placeholder user in the project' do
+    let!(:placeholder) { FactoryBot.create :placeholder_user, name: 'PLACEHOLDER' }
+    let(:wp_page) { Pages::FullWorkPackage.new(work_package) }
+
+    before do
+      login_as(user)
+      wp_page.visit_tab! :watchers
+    end
+
+    it 'should not show the placeholder user as an option' do
+      autocomplete = find('.wp-watcher--autocomplete')
+      target_dropdown = search_autocomplete autocomplete,
+                                            query: ''
+
+      expect(target_dropdown).to have_selector(".ng-option", text: user.name)
+      expect(target_dropdown).to have_no_selector(".ng-option", text: placeholder.name)
+    end
+  end
 end

spec/services/notifications/journal_wp_mail_service_spec.rb

@@ -102,6 +102,25 @@ describe Notifications::JournalWpMailService do
 
   it_behaves_like 'sends mail'
 
+  context 'assignee is placeholder user' do
+    let(:recipient) { FactoryBot.create :placeholder_user }
+
+    it_behaves_like 'sends no mail'
+  end
+
+  context 'responsible is placeholder user' do
+    let(:recipient) { FactoryBot.create :placeholder_user }
+    let(:work_package) do
+      FactoryBot.create(:work_package,
+                        project: project,
+                        author: author,
+                        responsible: recipient,
+                        type: project.types.first)
+    end
+
+    it_behaves_like 'sends no mail'
+  end
+
   context 'notification for work_package_added disabled' do
     let(:notification_setting) { %w(work_package_updated work_package_note_added) }