But the CL can't use some older wide angles(21,28) and collapsible can hit the sensor arm. Something like Dymo tape will limit how far the collapsible will go into the body though.
Same thing with the M5(not relevant)
the CL should not used leica collapsible lenses when they are collapsed but, interestingly enough, can use some voigtlander wide angle lenses -- I have a 15 I use all the time, and also a Canon 25 (very old) that works just wonderfully.
but, yeah, use a leica collapsible 50, leica says to put a circle of dynamo tape (or something similar) around the lens barrel to keep it from going into the body and hitting the meter.
FWIW, the 40mm Summicron C that comes with it is an excellent lens and can be used on M bodies as a snapshooter lens. It's not so odd, a lot of compacts used 40, and is a good compromise between wide angle and a normal lens that pokes out too far.