Change (apply 'nconc (mapcar ...)) to (mapcan ...); warn about first form. lisp/ChangeLog addition: 2010-09-16 Aidan Kehoe <kehoea@parhasard.net> * byte-optimize.el (byte-optimize-apply): Transform (apply 'nconc (mapcar ...)) to (mapcan ...); warn about use of the first idiom. * update-elc.el (do-autoload-commands): * packages.el (packages-find-package-library-path): * frame.el (frame-list): * extents.el (extent-descendants): * etags.el (buffer-tag-table-files): * dumped-lisp.el (preloaded-file-list): * device.el (device-list): * bytecomp-runtime.el (proclaim-inline, proclaim-notinline) Use #'mapcan, not (apply #'nconc (mapcar ...) in all these files. * bytecomp-runtime.el (eval-when-compile, eval-and-compile): In passing, mention that these macros also evaluate the body when interpreted.