Expand "to" of delete activities

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -305,11 +305,14 @@ def unfollow(follower, followed, activity_id \\ nil, local \\ true) do
   def delete(%Object{data: %{"id" => id, "actor" => actor}} = object, local \\ true) do
     user = User.get_cached_by_ap_id(actor)
 
+    to = object.data["to"] || [] ++ object.data["cc"] ||
+        [] ++ [user.follower_address, "https://www.w3.org/ns/activitystreams#Public"]
+
     data = %{
       "type" => "Delete",
       "actor" => actor,
       "object" => id,
-      "to" => [user.follower_address, "https://www.w3.org/ns/activitystreams#Public"]
+      "to" => Enum.uniq(to)
     }
 
     with {:ok, _} <- Object.delete(object),

test/web/activity_pub/activity_pub_test.exs

@@ -591,6 +591,16 @@ test "it creates a delete activity and deletes the original object" do
 
       assert Repo.get(Object, object.id).data["type"] == "Tombstone"
     end
+
+    test "it creates a delete activity and checks that it is also sent to users mentioned by the deleted object" do
+      user = insert(:user)
+      note = insert(:note_activity)
+      object = Object.get_by_ap_id(note.data["object"]["id"])
+      object = Kernel.put_in(object.data["to"], [user.ap_id])
+      {:ok, delete} = ActivityPub.delete(object)
+
+      assert user.ap_id in delete.data["to"]
+    end
   end
 
   describe "timeline post-processing" do