If you're thinking of adapting a Leica M compatible UWA (Leica, Voigtlander ot Zeiss) to an EOS R, forget it instantly.
Leica Ms have on the sides of the sensor specially oriented microlenses. Otherwise (the case with every ILC, except Leica SL 2), you'll see an ugly magenta or green cast on the sides of the picture. In my own experience, the only M lenses that can be used on an EOS R are lenses from 35 mm focal length and higher, as well as retrofocus WAs. UWAs specially designed for Leica Ms are not suited (no retrofocus design). No issues usually, with short or longer tele lenses.
In any case, test before buying!