* lisp/dired-aux.el (dired-diff): If file at point is a backup file,

use its original as the default value, and reverse the order
of arguments to the `diff' call.  Doc fix.

Fixes: debbugs:13772

3 files changed, 19 insertions, 6 deletions

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index 9721dba6b0f..44109a5095f 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,9 @@
+2013-02-28  Juri Linkov  <juri@jurta.org>
+
+	* dired-aux.el (dired-diff): If file at point is a backup file,
+	use its original as the default value, and reverse the order
+	of arguments to the `diff' call.  Doc fix.  (Bug#13772)
+
 2013-02-28  Michael Albinus  <michael.albinus@gmx.de>
 
 	* net/tramp-adb.el (tramp-adb-sdk-dir): Remove.  Replaced by ...
diff --git a/lisp/dired-aux.el b/lisp/dired-aux.el
index 13443419bd7..f6ff32b0b01 100644
--- a/lisp/dired-aux.el
+++ b/lisp/dired-aux.el
@@ -55,7 +55,8 @@ into this list; they also should call `dired-log' to log the errors.")
 (defun dired-diff (file &optional switches)
   "Compare file at point with file FILE using `diff'.
 If called interactively, prompt for FILE.  If the file at point
-has a backup file, use that as the default.  If the mark is active
+has a backup file, use that as the default.  If the file at point
+is a backup file, use its original.  If the mark is active
 in Transient Mark mode, use the file at the mark as the default.
 \(That's the mark set by \\[set-mark-command], not by Dired's
 \\[dired-mark] command.)
@@ -67,8 +68,10 @@ With prefix arg, prompt for second argument SWITCHES, which is
 the string of command switches for the third argument of `diff'."
   (interactive
    (let* ((current (dired-get-filename t))
-	  ;; Get the latest existing backup file.
-	  (oldf (diff-latest-backup-file current))
+	  ;; Get the latest existing backup file or its original.
+	  (oldf (if (backup-file-name-p current)
+		    (file-name-sans-versions current)
+		  (diff-latest-backup-file current)))
 	  ;; Get the file at the mark.
 	  (file-at-mark (if (and transient-mark-mode mark-active)
 			    (save-excursion (goto-char (mark t))
@@ -107,7 +110,10 @@ the string of command switches for the third argument of `diff'."
 		   (equal (expand-file-name current file)
 			  (expand-file-name current))))
       (error "Attempt to compare the file to itself"))
-    (diff file current switches)))
+    (if (and (backup-file-name-p current)
+	     (equal file (file-name-sans-versions current)))
+	(diff current file switches)
+      (diff file current switches))))
 
 ;;;###autoload
 (defun dired-backup-diff (&optional switches)
diff --git a/lisp/dired.el b/lisp/dired.el
index 28045043c82..f03e0aca475 100644
--- a/lisp/dired.el
+++ b/lisp/dired.el
@@ -3864,13 +3864,14 @@ Ask means pop up a menu for the user to select one of copy, move or link."
 ;;;;;;  dired-run-shell-command dired-do-shell-command dired-do-async-shell-command
 ;;;;;;  dired-clean-directory dired-do-print dired-do-touch dired-do-chown
 ;;;;;;  dired-do-chgrp dired-do-chmod dired-compare-directories dired-backup-diff
-;;;;;;  dired-diff) "dired-aux" "dired-aux.el" "066bb17769887a7fbc0490003f59e4b3")
+;;;;;;  dired-diff) "dired-aux" "dired-aux.el" "d6a694b6d13fa948465fde52a9ffb3ba")
 ;;; Generated autoloads from dired-aux.el
 
 (autoload 'dired-diff "dired-aux" "\
 Compare file at point with file FILE using `diff'.
 If called interactively, prompt for FILE.  If the file at point
-has a backup file, use that as the default.  If the mark is active
+has a backup file, use that as the default.  If the file at point
+is a backup file, use its original.  If the mark is active
 in Transient Mark mode, use the file at the mark as the default.
 \(That's the mark set by \\[set-mark-command], not by Dired's
 \\[dired-mark] command.)