MSC4123: Allow knock -> join transition

proposals/4123-knock-join-transition.md

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+# MSC4123: Allow `knock` -> `join` transition
+
+## Introduction
+
+[MSC2403](https://github.com/matrix-org/matrix-spec-proposals/pull/2403) added knocking functionality to
+rooms, with which came the membership state `knock`.
+At the time, `knock` -> `join` was not deemed as a valid transition, as for a user to join a room where you
+must knock, the knock must be accepted via an invite. (Although in cases where join rules were changed to
+be `public`, that would also be a sensible transition, but that is more of an edge case than the following)
+
+Later in [MSC3083](https://github.com/matrix-org/matrix-spec-proposals/pull/3083), restricted rooms were
+added as a way of joining rooms only when a given condition is met, most notabily when a user is a
+member of a given room.
+
+Finally, [MSC3787](https://github.com/matrix-org/matrix-spec-proposals/pull/3787) added the functionality
+to use both restricted join rules as well as knocking in a room simutainiously.
+However in this MSC the transition `knock` -> `join` was not added, even though it is very much a sensible
+transition with knock restricted rooms.
+For example, a user could initially knock on a room to join, but then later join a room required by the
+restricted join rule that could allow them to join directly without the knock being accepted.
+Currently, this would require the user to retract their knock, and then join, making the following transition:
+`knock` -> `leave` -> `join`.
+
+## Proposal
+
+In a new room version, we make `knock` -> `join` a valid transition, as it would make situations like the above
+easier to handle, as an unnecessary `knock` -> `leave` transition would no longer need to be sent.
+
+When it comes to authorizing a `knock` -> `join` transition, the same logic can be used as a `leave` -> `join`
+transition, as the same could be achieved with a `knock` -> `leave` -> `join` transition.
+
+## Potential issues
+
+Authorization would become *slightly* more complex, as developers would have to check against the room
+version to see if `knock` -> `join` is a valid transition, but it is a very minor change which would have
+the upside of not making redundant transitions, so it is worth it.
+
+## Alternatives
+
+Keep using the `knock` -> `leave` -> `join` transition instead, but the reasons this is undesirable
+are stated above.
+
+## Security considerations
+
+This MSC does not introduce anything that couldn't be done before (be it in a more complex and unintuitive way),
+so this should not have any security considerations.
+
+## Unstable prefix
+
+Implementations of this MSC should use `org.matrix.msc4123` for the room version, using room version 11 for all other
+behaviours of the room.
+
+## Dependencies
+
+None