Implement MSC 3818

Resolves: #11896

Signed-off-by: Aminda Suomalainen <suomalainen+git@mikaela.info>

Author: Mikaela

changelog.d/12786.feature

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

synapse/handlers/room.py

@@ -469,13 +469,19 @@ 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.
+        # Copy the room type as per MSC3818 https://github.com/matrix-org/matrix-spec-proposals/pull/3818.
+        if (
+            old_room_create_event.content.get(EventContentFields.ROOM_TYPE)
+            is not None
+        ):
+            creation_content[EventContentFields.ROOM_TYPE] =
+                old_room_create_event.content.get(EventContentFields.ROOM_TYPE)
+
+        # If the old room was a space, copy over 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))
 
         old_room_state_ids = await self.store.get_filtered_current_state_ids(