Say we want to test that an object is not referenced anywhere else, and use a weak list for that. (defun tester () (let ((wl (make-weak-list)) ;; Turn off GC. (gc-cons-threshold most-positive-fixnum)) ;; (cons 'foo 'bar) is a fresh object (set-weak-list-list wl (list (cons 'foo 'bar))) (assert (eq (length (weak-list-list wl)) 1)) (garbage-collect) (assert (eq (length (weak-list-list wl)) 0)))) Calling (tester) works as expected. However, after byte-compiling it with (byte-compile 'tester), the second assertion will fail. It seems that a pointer to the created object is left somewhere on the bytecode interpreter stack. If you add (setq x wl) somewhere in the function and garbage-collect after the function exits, (length (weak-list-list x)) will evaluate to 0. Inspecting bytecode.c proves this to be true -- all the stack is GCPRO'd all the time, so even discarded data is a "root of accessibility". I wonder if this can be construed as a bug. If yes, it could be fixed by having DISCARD either reset the discarded stuff to Qnil, or adjusting gcpro.nvars (I think the latter should be faster). On the other hand, I wouldn't want to affect the bytecode interpreter only to satisfy a very esoteric requirement. What do the others think?