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; emacs bug report.
; from Mike Rose, mrose@stsci.edu, 301-338-4949
;
; Problem when using tags-file-name as a local variable.
; m-. from a buffer with a local definition of tags-file-name
; that is different than the global definition returns the error
; message "Wrong type argument: bufferp, nil"

; This file is ~/bug.el and reproduces the problem.
; before running emacs, create to short tags files.
; The can have the same or different contents.
; The commands below create a tags file from this file,
; and then force the bug.
;     %cd ~
;     %etags -f TAGS-DEFAULT bug.el
;     %etags -f TAGS-LOCAL bug.el
;     %emacs -q
;         (load "~/bug")
; backtrace will show last call to verify-visited-file-modtime.
;
; The backtrace will look something like
; Signalling: (wrong-type-argument bufferp nil)
;   verify-visited-file-modtime(nil)
;   visit-tags-table-buffer()
;   find-tag("anything-but-dummy")
;   load("~/bug")
;   eval-region(1 15 #<buffer *scratch*>)
;   eval-print-last-sexp(nil)
; * call-interactively(eval-print-last-sexp)
; 
;
; My explanation of the problem:
; Neither of the two tags files is loaded when find-tag is from
; the buffer with a local definition of tags-file-name.  find-tag
; calls verify-visited-file-modtime, which does a set-buffer to
; a new buffer containing the tag file.  This new buffer has
; the global definition of tags-file-name.  Then, when eval-ing
; the arg for verify-visited-file-modtime, (get-file-buffer tags-file-name)
; is called and returns nil since tags-file-name is the global
; name, which names a file that has not been put in a buffer.
; Then (verify-visited-file-modtime nil) gives the error.


(defun dummy ())  ;for the tags files

(emacs-version)
;"GNU Emacs 18.55.0 of Thu Sep  6 1990 on kali.stsci.edu (berkeley-unix)"
(setq debug-on-error t)

(setq tags-file-name "~/TAGS-DEFAULT")
(switch-to-buffer (generate-new-buffer "foo"))
(make-local-variable 'tags-file-name)
(setq tags-file-name "~/TAGS-LOCAL")
(find-tag "anything-but-dummy")