type assertion too complex to check, unless inside another form

[Anticrisis]

I have been using SBCL's compile-time type checking and have run into an odd but easily avoided compiler note. I'm reporting a bug because it's hard for me to see why the workaround actually eliminates the note, unless it's either 1) suppressing a problem that should be noted, or 2) working around an erroneous note.

Simple to reproduce. The function TEST-1 generates a note when compiled, but the other functions do not. (SLIME output is embedded in the listing below.)

;;;; type-too-complex.lisp

(in-package #:cl-user)

;; SBCL 2.0.4

(deftype flag () '(member :a :b :c))
(deftype status () '(or flag (cons flag t)))

(defun alist-fetch (key list)
  (cdr (assoc key list)))

;; -----------------------------------------------------------------

(declaim (ftype (function (symbol list) (or null status)) test-1))
(defun test-1 (key alist-of-statuses)
  (alist-fetch key alist-of-statuses))

; compiling (DEFUN TEST-1 ...)

; file: C:/Users/anticrisis/AppData/Local/Temp/slime40
; in: DEFUN TEST-1
; (DEFUN TEST-1 (KEY ALIST-OF-STATUSES) (ALIST-FETCH KEY ALIST-OF-STATUSES))
; --> PROGN SB-IMPL::%DEFUN SB-IMPL::%DEFUN SB-INT:NAMED-LAMBDA
; ==>
; #'(SB-INT:NAMED-LAMBDA TEST-1
; (KEY ALIST-OF-STATUSES)
; (DECLARE (SB-C::TOP-LEVEL-FORM))
; (BLOCK TEST-1 (ALIST-FETCH KEY ALIST-OF-STATUSES)))
;
; note: type assertion too complex to check:
; (VALUES &OPTIONAL (OR (MEMBER :C :B :A NIL) (CONS (MEMBER :A :B :C) T)) &REST T).
;
; compilation unit finished
; printed 1 note

;; -----------------------------------------------------------------

(declaim (ftype (function (symbol list) (or null status)) test-2))
(defun test-2 (key alist-of-statuses)
  (let ((r (alist-fetch key alist-of-statuses))) r))

; compiling (DEFUN TEST-2 ...)

;; no notes printed

;; -----------------------------------------------------------------

(declaim (ftype (function (symbol list) (or null status)) test-3))
(defun test-3 (key alist-of-statuses)
  (values (alist-fetch key alist-of-statuses)))

; compiling (DEFUN TEST-3 ...)

;; no notes printed

[Anticrisis (anticrisis)]

Of course, seconds after submitting what I thought I had thoroughly reviewed, I noted that my report is incomplete: If the function ALIST-FETCH is also annotated with a function ftype declaration, all three TEST functions compile without a note. But I don't think that explains the variations in the original test case.

[Stas Boukarev (stassats)]

It complaints because your type declarations do not restrict the number of values. Threading the result through VALUES or a variable makes it a single value.

I improved the error message, now it says

; in: DEFUN FOO
; (THE FIXNUM (FUNCALL X))
;
; note: Type assertion too complex to check:
; FIXNUM.
; It allows an unknown number of values, consider using
; (VALUES FIXNUM &OPTIONAL).
;

[Anticrisis (anticrisis)]

Thank you for the quick action and for pointing me in the right direction. Every day I learn something!