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.