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From: Gregor@parc.xerox.com (Gregor Kiczales)
Newsgroups: comp.lang.clos
Subject: Re: Bug in May Day PCL (REV 4)
Message-ID: <91Feb8.103019pst.3664@anchor.parc.xerox.com>
Date: 8 Feb 91 18:30:12 GMT
References: <7533@ifi.informatik.uni-stuttgart.de>
Sender: welch@tut.cis.ohio-state.edu
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Organization: CommonLoops
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   Date: 	Thu, 7 Feb 1991 23:42:46 PST
   From: hohl%ifistg.uucp@ira.uka.de (Hubertus Hohl)

   Slot access in the new Revision 4 PCL [May Day PCL (REV 4)] does
   not always work correctly. Consider the following code fragment
   (extracted from a more complex piece of code):

      (defmethod m1 ((maybe-a-thing t))
	(if (typep maybe-a-thing 'thing)
	    (slot-value maybe-a-thing 't-1)
	    maybe-a-thing))

      (m1 (make-instance 'thing)) => 999

      (m1 'foo)  =>
      Error: No matching method for the generic-function #<Function FIND-SLOT-DEFINITION @ #x47fcbe>,
      when called with arguments (#<Built-In-Class SYMBOL 23723516>
T-1).

The bug here isn't with slot access, its with TYPEP.  PCL doesn't
redefine typep to know about CLOS classes.  You need to use the special
version of TYPEP provided by PCL.