There are certain Leica Summilux 35 1.4 non ASPH lenses that are incompatible with the M8 or M9. I have one that is made in Canada. I looked everywhere to find clues on how to fix the issue. I found little pieces of information and I wanted to do everyone else a favor by taking a video of how I resolved the issue.
First, to test if you have a problem - Set the focus to 3 feet (not to infinity). Mount the lens into your M8 or M9. Move the focus ring toward infinity GENTLY. If you feel resistance before infinity - stop. You have the issue. Now, to unmount, backup your focus ring to 3 feet again and then dismount. If you have the issue, do not try to mount the lens while focus is set to infinity. Bad.
The problem - There is a shroud that sticks out (a black piece of aluminum highlighted in my video) at the back of the lens. This shroud increases in height as you move to infinity. The M8 and M9 body is preventing that shroud to further stick out while moving to infinity. I have highlighted in the video exactly which part of the shroud is hitting. I also show how I've done the job myself of shaving 1mm off the shroud to allow the focus to get to infinity without obstruction. There are illustrations that say measure 7mm from the bottom of the ring to measure exactly how much to shave, but my video shows what I've done and it works for me. I'm now a happy camper. Happy enough to share what I did.
PS: I did not show it but I did tape the lens glass to protect it while I was grinding the shroud with my dremel tool.
Here is the video for future adventurers: