Implement MSC3818: copy room type on upgrade

changelog.d/12786.feature

@@ -0,0 +1 @@
+Implement [MSC3818: Copy room type on upgrade](https://github.com/matrix-org/matrix-spec-proposals/pull/3818).

synapse/handlers/room.py

@@ -469,14 +469,14 @@ async def clone_existing_room(
             (EventTypes.PowerLevels, ""),
         ]
 
-        # If the old room was a space, copy over the room type and the rooms in
-        # the space.
-        if (
-            old_room_create_event.content.get(EventContentFields.ROOM_TYPE)
-            == RoomTypes.SPACE
-        ):
-            creation_content[EventContentFields.ROOM_TYPE] = RoomTypes.SPACE
-            types_to_copy.append((EventTypes.SpaceChild, None))
+        # Copy the room type as per MSC3818.
+        room_type = old_room_create_event.content.get(EventContentFields.ROOM_TYPE)
+        if room_type is not None:
+            creation_content[EventContentFields.ROOM_TYPE] = room_type
+
+            # If the old room was a space, copy over the rooms in the space.
+            if room_type == RoomTypes.SPACE:
+                types_to_copy.append((EventTypes.SpaceChild, None))
 
         old_room_state_ids = await self.store.get_filtered_current_state_ids(
             old_room_id, StateFilter.from_types(types_to_copy)