+ ;; the function type which enables this transform.
+ ;;
+ ;; (Note that declaring this :TYPE FUN-TYPE probably wouldn't
+ ;; work because some function types, like (SPECIFIER-TYPE 'FUNCTION0
+ ;; itself, are represented as BUILT-IN-TYPE, and at least as of
+ ;; sbcl-0.pre7.54 or so, that's inconsistent with being a
+ ;; FUN-TYPE.)
+ (type (missing-arg) :type ctype)