This looks like an Emacs bug. Reduced reproducer:
$ cat repro.el (defsubst bar (a &rest b) nil) (defun foo () (bar 1 2 3 4)) $ emacs -Q --batch -f batch-byte-compile repro.el In toplevel form: repro.el:2:2: Error: Optimizer error: too much on the stack I'll see if I can make some workaround. -- Joel