gcontext extensions and broken code

Hi,

Does anyone know what COPY-GCONTEXT-COMPONENTS is meant to do?

SBCL's shiny new type inferencer has correctly detected that the code
implementing it currently invokes undefined behaviour for non-null
*GCONTEXT-EXTENSIONS*:

(defun copy-gcontext-components (src dst &rest keys)
  (declare (type gcontext src dst) (dynamic-extent keys))
  ...
  (multiple-value-bind (extension index)
		  (find key *gcontext-extensions* :key #'gcontext-extension-name)
		(if extension
		    (funcall (gcontext-extension-copy-function extension)
			     src dst (svref (gcontext-local-state src)
					    (index+ index *gcontext-data-length*)))
		  (x-type-error key 'gcontext-key)))
  ...)

Since FIND always returns one value, INDEX is always bound to NIL;
then INDEX+ acts on something that's declared to be a positive fixnum
(loosely).  The actual warning from SBCL says:

;     (XLIB::INDEX+ XLIB::INDEX XLIB::*GCONTEXT-DATA-LENGTH*)
; --> THE VALUES PROG1 LET + 
; ==>
;   (THE XLIB:ARRAY-INDEX XLIB::INDEX)
; 
; caught WARNING:
;   Asserted type (UNSIGNED-BYTE 29) conflicts with derived type
;   (VALUES NULL &OPTIONAL).

which I think is reasonable.

Any ideas what the correct code would look like?

Cheers,

Christophe
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