Performing a room upgrade on a space will not transition the type

[clokep]

If a room upgrade is performed on a space then the type field from the content will be lost.

We seem to pull across the m.federate content:

synapse/synapse/handlers/room.py

Lines 386 to 392 in a683028

	# Get old room's create event
	old_room_create_event = await self.store.get_create_event_for_room(old_room_id)
	# Check if the create event specified a non-federatable room
	if not old_room_create_event.content.get("m.federate", True):
	# If so, mark the new room as non-federatable as well
	creation_content["m.federate"] = False

But I wonder if we should start with the old event's content and then splat room_version and predecessor on top of it?

The spec is a bit vague, but seems to imply we shouldn't even be copying m.federate:

Creates a replacement room with a m.room.create event containing a predecessor field and the applicable room_version.

We also will want to copy the m.space.child and m.space.parent events from the space. It is unclear what we would do with the m.space.parent events in rooms that point to the space.

This might need an MSC for discussion, or might just be an implementation detail.

[clokep]

@turt2live and I had a discussion about this in matrix-spec: https://matrix.to/#/!NasysSDfxKxZBzJJoE:matrix.org/$ia7ZPMbF4BazNtABrnBOSxmUQCeNufcJFYcn_0-pdFs?via=matrix.org&via=libera.chat&via=vector.modular.im

The conclusion was that the spec purposefully leaves this vague and implementation dependent.

I think what we should probably do is: If room_type = m.space, then copy any m.space.child events over.