And, oh look, something nasty did happen to the guys wearing red.
* Krystof sighs at the irony in CLHS 22.214.171.124 <pfdietz> And its interaction with the rules for upgraded array element types? * pfdietz realizes that those bizarre NIL arrays are strings!
Most Common Lisp implementations purporting to conform to the standard have only one internal representation for strings: it simplifies the logic; it allows for fast string access (no need to do a type test to see how big each string element); it makes type derivation easy. Several implementations can talk different representations, but internally, they're canonicalized to ASCII, UCS-16, UCS-32 or whatever.
Sadly, since we've previously deduced that arrays specialized to hold no objects must exist, and since strings are defined to be vectors specialized to hold a subtype of character, vectors specialized to hold objects of the NIL type are indeed strings.
Aargh. I have a working prototype, but there are going to be plenty of issues involved in sorting this out. At least they're mostly the same issues as would be involved in dealing with the Unicodization of SBCL, so it's not wasted work. It might even encourage me to do some more refactoring to share more code among the six backends.