Well firstly, the EF-M will not fit on EF, nor EF-s. EF-M sensor to lens distance is 18mm, EF and EF-s are both 44mm. (with the usual disclaimer that maybe it could fit with an adapter that means it won't focus to infinity, or it could also fit with an adapter full of glass that means it will focus to infinity, but not look very good).

The thing with Pancakes in general is that they usually perform best where the focal length is about the same as the flange distance. Hence 40mm pancake, 44mm flange. Or 22mm pancake, 18mm flange. Even on 6x6 i've got an 80mm Biometar (Planar) lens with 75mm flange (well, it's not exactly pancake, it's the size of a Nifty Fifty, but that's pretty small for 6x6 and it's about the same size as a teleconverter).
There's a lot of other pancakes for different brands, Olympus had an OM-mount 40mm f/2 that was physically 25mm long, and a fair few for other mirrorless systems.

All that said, Pentax do have a very wide pancake, the 21mm f/3.2, still only 25mm long, which is a fairly good achievement considering how retrofocus it has to be to get that wide (although the IQ isn't as good as the Shorty Forty, it's not bad, and it only fits Crop cameras). They've also got a 40mm f/2.8 Pancake and a 43mm f/1.9.

So yeah, if Pentax can do it (make a wide-angle pancake for dslr), then i'm sure Canon can too, if they wanted to. Canon seems to have an aversion to making EF-S primes though, especially in everyday-useful lengths, so we'll probably never see one, and I just think that ~20mm is too wide to make a useful FF-pancake.
Maybe we'll get lucky and they'll give us a 28mm or 30mm pancake too?