Don't allow category definition topics to be converted to PMs (#5216)

app/assets/javascripts/discourse/widgets/topic-admin-menu.js.es6

@@ -177,7 +177,7 @@ export default createWidget('topic-admin-menu', {
                    icon: visible ? 'eye-slash' : 'eye',
                    label: visible ? 'actions.invisible' : 'actions.visible' });
 
-    if (this.currentUser.get('staff')) {
+    if (details.get('can_convert_topic')) {
       buttons.push({ className: 'topic-admin-convert',
                      action: isPrivateMessage ? 'convertToPublicTopic' : 'convertToPrivateMessage',
                      icon: isPrivateMessage ? 'comment' : 'envelope',

app/serializers/topic_view_serializer.rb

@@ -118,6 +118,7 @@ class TopicViewSerializer < ApplicationSerializer
     result[:can_create_post] = true if scope.can_create?(Post, object.topic)
     result[:can_reply_as_new_topic] = true if scope.can_reply_as_new_topic?(object.topic)
     result[:can_flag_topic] = actions_summary.any? { |a| a[:can_act] }
+    result[:can_convert_topic] = true if scope.can_convert_topic?(object.topic)
     result
   end
 

lib/guardian/topic_guardian.rb

@@ -63,7 +63,9 @@ module TopicGuardian
   end
 
   def can_convert_topic?(topic)
+    return false if topic.blank?
     return false if topic && topic.trashed?
+    return false if Category.where("topic_id = ?", topic.id).exists?
     return true if is_admin?
     is_moderator? && can_create_post?(topic)
   end

spec/components/guardian_spec.rb

@@ -996,6 +996,12 @@ describe Guardian do
       expect(Guardian.new(trust_level_4).can_convert_topic?(topic)).to be_falsey
     end
 
+    it 'returns false for category definition topics' do
+      c = Fabricate(:category)
+      topic = Topic.find_by(id: c.topic_id)
+      expect(Guardian.new(admin).can_convert_topic?(topic)).to be_falsey
+    end
+
     it 'returns true when a moderator' do
       expect(Guardian.new(moderator).can_convert_topic?(topic)).to be_truthy
     end