;; The function SYNTAX-ERROR is supposed to not return, eg (function (&rest t) nil) ?
(labels ((syntax-error (&rest stuff)
           (error "Syntax error: ~s" stuff))

         ;; This function is fine on its own without read-something
         (get-num (key)
           (the fixnum (case key
                         (:a 1)
                         (:b 2)
                         (otherwise (syntax-error "Hey, key was wrong" key)))))

         ;; The presence of READ-FUNCTION makes SBCL wrongly think this about the above usage:
         ;;     Derived type of (SYNTAX-ERROR "Hey, key was wrong" KEY) is
         ;;       (VALUES (MEMBER :TRUE) &OPTIONAL),

         (read-something (key)
           (if (eq key :foo)
               :true
               (syntax-error "not :foo"))))
  
  ;; The presence or non-presence of this declaration changes nothing
  (declare (ftype (function (&rest t) nil) syntax-error))
        
  (defun test (key)
    ;; dumb calls
    (get-num key)
    (read-something key)))