SUBTYPEP confusion re. OR of SATISFIES of not-yet-defined predicate

Bug #308938 reported by Nikodemus Siivola on 2008-12-17
Affects Status Importance Assigned to Milestone

Bug Description

 As reported by Levente M\'{e}sz\'{a}ros sbcl-devel 2006-02-20,
  (aver (equal (multiple-value-list
                       (subtypep '(or (satisfies x) string)
                                 '(or (satisfies x) integer)))
                      '(nil nil)))
 fails. Also, beneath that failure lurks another failure,
  (aver (equal (multiple-value-list
                              (subtypep 'string
                                 '(or (satisfies x) integer)))
               '(nil nil)))
 Having looked at this for an hour or so in sbcl-1.0.2, and
 specifically having looked at the output from
   laptop$ sbcl
   * (let ((x 'string)
           (y '(or (satisfies x) integer)))
       (trace sb-kernel::union-complex-subtypep-arg2
       (subtypep x y))
 my (WHN) impression is that the problem is that the semantics of TYPE=
 are wrong for what the UNION-COMPLEX-SUBTYPEP-ARG2 code is trying
 to use it for. The comments on the definition of TYPE= probably
 date back to CMU CL and seem to define it as a confusing thing:
 its primary value is something like "certainly equal," and its
 secondary value is something like "certain about that certainty."
 I'm left uncertain how to fix UNION-COMPLEX-SUBTYPEP-ARG2 without
 reducing its generality by removing the TYPE= cleverness. Possibly
 the tempting TYPE/= relative defined next to it might be a
 suitable replacement for the purpose. Probably, though, it would
 be best to start by reverse engineering exactly what TYPE= and
 TYPE/= do, and writing an explanation which is so clear that one
 can see immediately what it's supposed to mean in odd cases like
 (TYPE= '(SATISFIES X) 'INTEGER) when X isn't defined yet.

Changed in sbcl:
importance: Undecided → Medium
status: New → Confirmed
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