bogus type warning

[Tobias C. Rittweiler]

Compiling the following

  (defun frob (timeout)
    (let ((start-time (and timeout (get-internal-real-time))))
      (when timeout
        (- (get-internal-real-time) start-time))))

results in

  ; in: DEFUN FROB
  ; (AND TIMEOUT (GET-INTERNAL-REAL-TIME))
  ; --> IF
  ; ==>
  ; NIL
  ;
  ; caught WARNING:
  ; This is not a NUMBER:
  ; NIL
  ; See also:
  ; The SBCL Manual, Node "Handling of Types"
  ;
  ; compilation unit finished
  ; caught 1 WARNING condition

I think that's the same issue as the infamous CL-PPCRE + SBCL annoyance
where some macro expands to similiar code, causing SBCL to emit warnings.

Paul worked on something to fix it, IIRC.

It's similiar to Bug #309451 however that talks about downgrading to style-
warning that I do not see.