As expected, the Quicklisp distribution of LET-OVER-LAMBDA is broken by the changes to the backquote reader macro in SBCL 1.2.2; although I expect this change breaks a good portion of Paul Graham’s macro code examples from On Lisp, as well.
A quick-fix suggested on Reddit is to use a “pseudo-flatten” for SBCL that also descends into
sb-impl::comma. I will be testing this fix today, and hopefully pushing an updated version for the August release of Quicklisp.
LOL:FLATTEN to descend into
sb-impl::comma objects, but no joy. I have currently disabled
DEFMACRO! based code in LET-OVER-LAMBDA until I can find a better solution, and tested this against both v1.2.2 and v1.2.0-1 of SBCL (so if nothing else, it will at least build without errors).
If anyone knows of a better solution, feel free to leave a comment here or on the GitHub Issue thread.