Trying to accept invite from user who has already left room fails badly

[ara4n]

Steps to reproduce

1. Get an invite
2. Try to accept it
3. Get "Sorry, an error occurred".

I'm assuming that the problem is that the user invited me, then left the room.

Outcome

What did you expect?

Rooms shouldn't wink out of existence if they have nobody in them: https://github.com/matrix-org/matrix-spec/issues/95

However, accepting an invite to an empty room could be pretty annoying too, especially if it had no history in it.

Alternative solutions could be:

• Automatically withdraw invites sent by users when the inviter parts the room (although it's possible that sometimes this behaviour would be desirable - e.g. an invite bot which joins a room, invites someone, and then leaves it)
• Automatically withdraw invites in a room when all the joined users part (so that it winks out of existence fully)
• Don't let users exit rooms without first withdrawing any invites they sent (i.e. force the client to clean up)

What happened instead?

Awful UX.

Your phone model

No response

Operating system version

No response

Application version

475

Homeserver

No response

Will you send logs?

No

[giomfo]

We may need to fix https://github.com/element-hq/synapse/issues/4808 before fixing this issue