MAPCAN/MAPCON Edge cases

[Gleefre]

MAPCAN/MAPCON functions have two edge cases which are not correctly handled in abcl.

(mapcan #'identity '(1 2 3))  ; should error, returns 3 instead
(mapcon #'car '(1 2 3))       ; same

(setf *print-circle* t)

;; This returns #1=(1 2 . #1#), which is correct
(let ((lst (list 1 2)))
  (flet ((f (x)
           (declare (ignore x))
           lst))
    (apply #'nconc (mapcar #'f '(1 2)))))

;; Should be equivalent to the previous example, but hangs instead.
(let ((lst (list 1 2)))
  (flet ((f (x)
           (declare (ignore x))
           lst))
    (mapcan #'f '(1 2))))

Also please see https://gitlab.common-lisp.net/ansi-test/ansi-test/-/issues/39 .

Tested on abcl 1.9.1 (latest release)

[easye]

Fairly complicated reasoning about what should occur in https://gitlab.common-lisp.net/cmucl/cmucl/-/issues/196, for which there doesn't appear to be a clear consensus.