News from the God of Camera Lens Cross-Adapters: an adapter from SLR bayonet to Leica M with rangefinder coupling. Crazy: you focus with the adapter and read the distance from the adapter and transfer this distance setting identically to the SLR lens. So only a half coupling, but better than nothing.
Okay, I wouldn't want to focus a 100/2.0 from Zeiss on my M at full aperture, but for the usual suspects this whole thing could be quite practical. You save the purchase of a rather expensive Leica lens in some cases.
On the other hand, the adapter costs $325, which is a bit much just to use such exotic combinations. And in some cases it's simply better to get the cheaper Voigtländer lenses.
The idea is just so crazy that it's good again.