Consider joined guest users as joined users

Otherwise they're inconveniently allowed to write events to the room
but not to read them from the room.

synapse/handlers/message.py

@@ -258,7 +258,19 @@ class MessageHandler(BaseHandler):
 
     @defer.inlineCallbacks
     def _check_in_room_or_world_readable(self, room_id, user_id, is_guest):
-        if is_guest:
+        try:
+            # check_user_was_in_room will return the most recent membership
+            # event for the user if:
+            #  * The user is a non-guest user, and was ever in the room
+            #  * The user is a guest user, and has joined the room
+            # else it will throw.
+            member_event = yield self.auth.check_user_was_in_room(room_id, user_id)
+            defer.returnValue((member_event.membership, member_event.event_id))
+            return
+        except AuthError:
+            if not is_guest:
+                raise
+
         visibility = yield self.state_handler.get_current_state(
             room_id, EventTypes.RoomHistoryVisibility, ""
         )
@@ -269,9 +281,6 @@ class MessageHandler(BaseHandler):
             raise AuthError(
                 403, "Guest access not allowed", errcode=Codes.GUEST_ACCESS_FORBIDDEN
             )
-        else:
-            member_event = yield self.auth.check_user_was_in_room(room_id, user_id)
-            defer.returnValue((member_event.membership, member_event.event_id))
 
     @defer.inlineCallbacks
     def get_state_events(self, user_id, room_id, is_guest=False):