DON'T !!!!Sadly I just sold my EF20 2.8 -- what great lens! I found it big with the adapter and I usually shoot it manually.
Now I shoot a Voigtlander 21/4 Skopar until I find a good Leica 21mm that is not over-priced.
I have the 21 Leica M lens (non-aspherical).
Mounted on the EOS R, it generates a reddish hue on the sides of the pictures, like all M lenses below 35 mm, you can even see it in the viewfinder!
The Leica sensors have therefore specifically oriented microlenses against this phenomenon.
Maybe (?) the asph. type behaves differently, even though I doubt it.
PS: the Leica R 21 mm is not really good...