* lisp/dired-aux.el (dired-diff): Restore original functionality of

getting the default value, but keep new feature of using the
latest existing backup file (`diff-latest-backup-file').

etc/NEWS

@@ -468,8 +468,8 @@ file at point.
 mark/unmark/flag all files in the active region.
 
 *** The minibuffer default for `=' (`dired-diff) has changed.
-It is now the backup file for the file at point, if one exists, rather
-than the file at the mark.
+It is now the backup file for the file at point, if one exists.
+In Transient Mark mode the default is the file at the active mark.
 
 *** `M-=' is no longer bound to `dired-backup-diff' in Dired buffers.
 The global binding for `M-=', `count-words-region' is in effect.

lisp/ChangeLog

+2012-09-18  Juri Linkov  <juri@jurta.org>
+
+	* dired-aux.el (dired-diff): Restore original functionality of
+	getting the default value, but keep new feature of using the
+	latest existing backup file (`diff-latest-backup-file').
+
 2012-09-18  Juri Linkov  <juri@jurta.org>
 	* dired.el (dired-mark): If the region is active in Transient Mark

lisp/dired-aux.el

@@ -51,33 +51,57 @@ into this list; they also should call `dired-log' to log the errors.")
 (defconst dired-star-subst-regexp "\\(^\\|[ \t]\\)\\*\\([ \t]\\|$\\)")
 (defconst dired-quark-subst-regexp "\\(^\\|[ \t]\\)\\?\\([ \t]\\|$\\)")
 
+(declare-function diff-latest-backup-file "diff" (fn)) ; actually belongs into files.el
+
 ;;;###autoload
 (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 called interactively, prompt for FILE.  If the file at point
+has a backup file, use that as the default.  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.)
+
+FILE is the first file given to `diff'.  The file at point
+is the second file given to `diff'.
 
-FILE is the first file given to `diff'.
-With prefix arg, prompt for second argument SWITCHES,
-which is the string of command switches for `diff'."
+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))
-	  (oldf (file-newest-backup current))
-	  (dir (if oldf (file-name-directory oldf))))
-     (list (read-file-name
-	    (format "Diff %s with%s: "
-		    (file-name-nondirectory current)
-		    (if oldf
-			(concat " (default "
-				(file-name-nondirectory oldf)
-				")")
-		      ""))
-	    dir oldf t)
-	   (if current-prefix-arg
-	       (read-string "Options for diff: "
-			    (if (stringp diff-switches)
-				diff-switches
-			      (mapconcat 'identity diff-switches " ")))))))
+	  ;; Get the latest existing backup file.
+	  (oldf (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))
+					    (dired-get-filename t t))))
+	  (default-file (or file-at-mark
+			    (and oldf (file-name-nondirectory oldf))))
+	  ;; Use it as default if it's not the same as the current file,
+	  ;; and the target dir is current or there is a default file.
+	  (default (if (and (not (equal default-file current))
+			    (or (equal (dired-dwim-target-directory)
+				       (dired-current-directory))
+				default-file))
+		       default-file))
+	  (target-dir (if default
+			  (dired-current-directory)
+			(dired-dwim-target-directory)))
+	  (defaults (dired-dwim-target-defaults (list current) target-dir)))
+     (list
+      (minibuffer-with-setup-hook
+	  (lambda ()
+	    (set (make-local-variable 'minibuffer-default-add-function) nil)
+	    (setq minibuffer-default defaults))
+	(read-file-name
+	 (format "Diff %s with%s: " current
+		 (if default (format " (default %s)" default) ""))
+	 target-dir default t))
+      (if current-prefix-arg
+	  (read-string "Options for diff: "
+		       (if (stringp diff-switches)
+			   diff-switches
+			 (mapconcat 'identity diff-switches " ")))))))
   (let ((current (dired-get-filename t)))
     (when (or (equal (expand-file-name file)
 		     (expand-file-name current))

lisp/dired.el

@@ -3763,17 +3763,22 @@ 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" "3650b53533253c50b10e2aa8c9005ebf")
+;;;;;;  dired-diff) "dired-aux" "dired-aux.el" "2a883f0d481a8d0292eb90c09ae36a8e")
 ;;; 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 called interactively, prompt for FILE.  If the file at point
+has a backup file, use that as the default.  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.)
 
-FILE is the first file given to `diff'.
-With prefix arg, prompt for second argument SWITCHES,
-which is the string of command switches for `diff'.
+FILE is the first file given to `diff'.  The file at point
+is the second file given to `diff'.
+
+With prefix arg, prompt for second argument SWITCHES, which is
+the string of command switches for the third argument of `diff'.
 
 \(fn FILE &optional SWITCHES)" t nil)