You mention many times that you can wet vent your shower by attaching it to your vented lav drain. Are there any problems doing it the other way around, venting your lav by attaching it to your vented shower drain?

You mention many times that you can wet vent your shower by attaching it to your vented lav drain. Are there any problems doing it the other way around, venting your lav by attaching it to your vented shower drain?
That won't work because you will be building a outlawed "S" trap in the lavatory trap. Regards, Tom