ChristopherMarkPerez said:
Simple optical physics says a 50mm lens measures 50mm's from the first optical element to imaging plane (film or sensor, it doesn't matter which). That's just under 2 inches. Is that pancake enough?
In the days of DSLRs with their mirrorboxes, (slightly) inverted telephoto designs meant the length from front to back of a lens could be rather long. In these cases, complex optical designs mean that the position of the first element is many times well beyond 50mm's.
Well, i would certainly expect a potential EF-M 50/1.8 IS STM to be closer in size to the EF 50/1.8 STM (l=39.3mm, dia=69.2, filter 49mm) than to a Zeiss Otus.

Question is, whether the reduced image circle (EF is full frame, M only APS-C) would allow an optical design that offsets the size impact stemming from difference in flange distance. Don't know, but hope it would be possible. Those advanced optical design apps are pretty powerful today.