At the time of writing, for an individual UpdateOpEntry, it is an error to specify both 'multi: true' and a sort. This check is enforced pretty close to runtime (i.e. before we execute the update) on both mongod and mongos. Ideally, this would be checked at parse time. 

As such, this ticket tracks the work to move this check to parse time. Note that UpdateOpEntry is parsed using IDL, so this is effectively asking whether it is possible to add a custom parsing rule or a mutual exclusion rule to the existing IDL.