You didn't answer the question if SBCL infers specialized list types. I'll assume the answer is "no". In that case, everything extracted from a list is T and y

You didn't answer the question if SBCL infers specialized list types. I'll assume the answer is "no". In that case, everything extracted from a list is T and you'd have to get that warning you mentioned above. Such warnings would very likely be false-positive, unless Common Lisp programs are buggy as hell most of the time—assuming that would be the real FUD, no? Therefore, the non-FUD conclusion is to expect a high rate of false-positive warnings in typical list-using programs. TBH, I suspect those false-positive warnings you mentioned aren't bona fide warnings, but some kind optimizer stream-of-consciousness log stream.