(declaim (ftype (function () (values &optional nil)) my-error))
(defun my-error ()
  (error "Oops"))

(defvar *a* t)

(defun foo ()
  (if *a*
      (values 1 2 3)
      (my-error)))

;; ; SLIME 2.29.1
;; ; file: /home/death/tmp/foo.lisp
;; ; in: DEFUN FOO
;; ;     (VALUES 1 2 3)
;; ; 
;; ; caught STYLE-WARNING:
;; ;   Result is a (VALUES (INTEGER 1 1) (INTEGER 2 2) (INTEGER 3 3) &OPTIONAL), 
;; ;            not a (VALUES &OPTIONAL).
;; ;   See also:
;; ;     The SBCL Manual, Node "Handling of Types"
;; ; 
;; ; compilation unit finished
;; ;   caught 1 STYLE-WARNING condition

;; CL-USER> (foo)
;; ; No value
;; CL-USER>