Fix 'list-tags' when invoked from a non-file buffer

This use case was broken by the improvement that attempts to
offer the current buffer's file name as the default file whose
tags to list.
* lisp/progmodes/etags.el
(tags--get-current-buffer-name-in-tags-file): Doc fix.  Return nil
if no file is associated with the current buffer, and avoid
signaling an error if 'buffer-file-name' returns nil.  (Bug#37611)
(list-tags): Doc fix.  Signal an error if the user specifies no
file name at the prompt.

* doc/emacs/maintaining.texi (List Identifiers): Fix wording of
the documentation of 'list-tags'.

2 files changed, 32 insertions, 20 deletions

diff --git a/doc/emacs/maintaining.texi b/doc/emacs/maintaining.texi
index 3c34afbaa20..a632ffda4ab 100644
--- a/doc/emacs/maintaining.texi
+++ b/doc/emacs/maintaining.texi
@@ -2664,14 +2664,13 @@ loaded, this command can use it to generate completion candidates.
 @xref{Symbol Completion}.
 
 @findex list-tags
-  @kbd{M-x list-tags} reads the name of one of the files covered by
-the selected tags table, and displays a list of tags defined in that
-file.  Do not include a directory as part of the file name unless the
-file name recorded in the tags table includes a directory.  This
-command works only with the etags backend, and requires a tags table
-for the project to be available.  @xref{Tags Tables}.  If used
-interactively, the default tag is file name of the current buffer if
-used interactively.
+  @kbd{M-x list-tags} reads the name of one of the files covered by the
+selected tags table, with completion, and displays the list of tags
+defined in that file; it offers the current buffer's file name as the
+default file whose tags to list.  Do not include a directory as part of
+the file name unless the file name recorded in the tags table includes a
+directory.  This command works only with the etags backend, and requires
+a tags table for the project to be available.  @xref{Tags Tables}.
 
 @findex tags-next-file
   @kbd{M-x tags-next-file} visits files covered by the selected tags table.
diff --git a/lisp/progmodes/etags.el b/lisp/progmodes/etags.el
index d3eb0d46e9b..35dc0215046 100644
--- a/lisp/progmodes/etags.el
+++ b/lisp/progmodes/etags.el
@@ -1894,27 +1894,40 @@ description of the arguments."
       (try-completion string (tags-table-files) predicate))))
 
 (defun tags--get-current-buffer-name-in-tags-file ()
-  "Get the file name that the current buffer corresponds in the tags file."
-  (let ((tag-dir
-         (save-excursion
-           (visit-tags-table-buffer)
-           (file-name-directory (buffer-file-name)))))
-    (file-relative-name (buffer-file-name) tag-dir)))
+  "Return file name that corresponds to the current buffer in the tags table.
+This returns the file name which corresponds to the current buffer relative
+to the directory of the current tags table (see `visit-tags-table-buffer').
+If no file is associated with the current buffer, this function returns nil."
+  (let ((buf-fname (buffer-file-name)))
+    ;; FIXME: Are there interesting cases where 'buffer-file-name'
+    ;; returns nil, but there's some file we expect to find in TAGS that
+    ;; is associated with the buffer?  The obvious cases of Dired and
+    ;; Info buffers are not interesting for TAGS, but are there any
+    ;; others?
+    (if buf-fname
+        (let ((tag-dir
+               (save-excursion
+                 (visit-tags-table-buffer)
+                 (file-name-directory buf-fname))))
+          (file-relative-name buf-fname tag-dir)))))
 
 ;;;###autoload
 (defun list-tags (file &optional _next-match)
   "Display list of tags in file FILE.
-This searches only the first table in the list, and no included
-tables.  FILE should be as it appeared in the `etags' command,
-usually without a directory specification.  If called
-interactively, FILE defaults to the file name of the current
-buffer."
+Interactively, prompt for FILE, with completion, offering the current
+buffer's file name as the defaul.
+This command searches only the first table in the list of tags tables,
+and does not search included tables.
+FILE should be as it was submitted to the `etags' command, which usually
+means relative to the directory of the tags table file."
   (interactive (list (completing-read
                       "List tags in file: "
                       'tags-complete-tags-table-file
                       nil t
-                      ;; Default FILE to the current buffer.
+                      ;; Default FILE to the current buffer's file.
                       (tags--get-current-buffer-name-in-tags-file))))
+  (if (string-empty-p file)
+      (user-error "You must specify a file name"))
   (with-output-to-temp-buffer "*Tags List*"
     (princ (substitute-command-keys "Tags in file `"))
     (tags-with-face 'highlight (princ file))