FIX: handle moving first post to a closed/archived topic

app/models/post_mover.rb

@@ -90,7 +90,8 @@ class PostMover
       raw: post.raw,
       topic_id: destination_topic.id,
       acting_user: user,
-      skip_validations: true
+      skip_validations: true,
+      guardian: Guardian.new(user)
     )
 
     PostAction.copy(post, new_post)

spec/models/post_mover_spec.rb

@@ -391,6 +391,19 @@ describe PostMover do
         end
       end
 
+      context "to an existing closed topic" do
+        let!(:destination_topic) { Fabricate(:topic, closed: true) }
+
+        it "works correctly for admin" do
+          admin = Fabricate(:admin)
+          moved_to = topic.move_posts(admin, [p1.id, p2.id], destination_topic_id: destination_topic.id)
+          expect(moved_to).to be_present
+
+          moved_to.reload
+          expect(moved_to.posts_count).to eq(2)
+          expect(moved_to.highest_post_number).to eq(2)
+        end
+      end
 
     end
   end