Membership should be transferred when upgrading a room

[turt2live]

Joins should be automatic where possible. Invite-only rooms are a bit of a questionable one though (https://github.com/matrix-org/matrix-doc/issues/1864).

For joins, we might be able to work around the complexity by abusing Matrix: when upgrading the room, send invites to all the current members and servers can be expected to auto-accept following the verification of the invite being for an upgraded room. Or possibly not auto-accept?

Starting point for the discussion is here: matrix-org/matrix-spec-proposals#1930 (comment)

[ara4n]

This should be split into two halves: the common case of autojoining a public room (which is easy), and the second phase of autojoining an invite-only only room (which is more fiddly as we need to autoaccept the users somehow).

It's worth noting that until both halves are done, people are going to miss room upgrades unless matrix-org/matrix-spec-proposals#1930 becomes a thing.

[clokep]

the second phase of autojoining an invite-only only room (which is more fiddly as we need to autoaccept the users somehow)

MSC2214 and MSC3325 attempt to tackle this. I think both proposals have some minor issues with sharing keys for encrypted messages, however.