Zv said:
Am I right in assuming that current EF-M lenses won't cover a full frame sensor if it were possible to fit one into an EOS M today
Probably, but you may well be surprised. One interesting experiment is to take standard Canon EF-S lenses and, with the metabones adaptor, use them on the Sony A7RII FF mirrorless. By default it recognises these are APS-C lenses and crops down accordingly (ie 18mpx 1.5x crop rather than 40mpx full frame), but in the menu you can force it to use the full sensor and then we see some interesting things.
Most lenses I tried comfortably fill a much greater area than the APS-C rectangle (albeit with decreased sharpness and vignetting). The venerable EF-S 18-55 kit lens almost fills the full frame sensor at 24mm with just a tiny crop at the corners!
Certainly if you had an EF-M lens and could fit it on a FF mirrorless camera I wouldn't expect FF coverage in every case, but I'd be surprised if you didn't get more than the APS-C area.
If I were Canon, I'd announce a new Mirrorless FF camera as (assuming same sensor size as A7RII) a 18/40mpx camera stating that the standard resolution is 18mpx, with 40mpx full frame available only with suitable lenses.
That way there would be much less confusion.
Also.... It's not like Canon has a huge range of EF-M lenses that would become obsolete right now...