In the method browser, the function COMPUTE-APPLICABLE-METHODS-FROM-SPECIALIZERS determines the applicable methods by first creating a list of prototype instances for the specializers. The instances are then fed to COMPUTE-APPLICABLE-METHODS.
The problem is that fetching prototype instances of built-in classes is brittle (the function is not part of ANSI), e.g. in Allegro:
CL-USER(78): (mop::class-prototype (find-class 'integer)) Error: The slot EXCL::PROTOTYPE is missing from the object #<BUILT-IN-CLASS INTEGER> of class #<STANDARD-CLASS BUILT-IN-CLASS> during operation SLOT-VALUE [condition type: PROGRAM-ERROR]
Restart actions (select using :continue): 0: Try accessing the slot again 1: Return a value 2: Return to Top Level (an "abort" restart). 3: Abort entirely from this (lisp) process.
See also this discussion: http://groups.google.nl/group/comp.lang.lisp/browse_frm/thread/a15fd265fa3b5...
The attached patch makes the method browser robust against unavailable prototype instances, and documents where things still to wrong.
Cheers, - Willem