Hello Frederic,
Thank you for the patch! Afair problem has already been reported in SBCL and fixed in their repository. Updating your compiler may make this thing go away. We had some reports about it already.
Regarding the change – code as it is now is cleaner and it is not very likely that we will change it to hide a bug in a compiler. That said if you are interested in cleaning up this patch and submitting in a form of a pull request to https://github.com/McCLIM/McCLIM it may get accepted (if it will be more straightforward than the current code).
I am very glad you enjoy using McCLIM. If you have an interesting project to show written in it don't hestitate to share it on this mailing list!
Best regards, Daniel