Fix alphabetic sorting for headlines, tags

* org.el (org-sort-entries): Use collated sorting.
(org-tags-sort-function): Use collated sorting.
(org-string-collate-greaterp): Add helper-function to use as defcustom
option, since there is no ‘string-collate-greaterp’ in Emacs.

* org-compat.el (org-string-collate-lessp): Add proxy to fall-back on
string-lessp when string-collate-lessp is missing (Emacs ≤ 24).

* test-org.el (test-org/string-collate-lessp): Add test.
(test-org/sort-entries): Add regression test for non-ASCII inputs.

‘org-sort-entries’ and ‘org-tags-sort-function’ advertise alphabetic
sorting, but actually sort based only on character code.  This
produces non-alphabetic orderings of strings in non-ASCII locales.

E. g., German Umlauts “Ä Ü Ö” are alphabetically sorted as if they
were “A U O”, whereas sorting based on character-code will place them
after “Z”, which is unexpected.

etc/ORG-NEWS

@@ -243,6 +243,11 @@ of these exporters will properly export to =irc:= links, which will
 open properly in irc clients from web browsers.
 
 *** ~org-comment-dwim~ (bound to =M-;=) now comments headings, if point is on a heading
+*** Alphabetic sorting in headings and tags now uses the locale’s sorting rules
+
+When sorting alphabetically, ~org-sort-entries~ and
+~org-tags-sort-function~ now sort according to the locale’s collation
+rules instead of by code-point.
 * Version 9.1
 
 ** Incompatible changes

lisp/org-compat.el

@@ -118,6 +118,12 @@ output directories whose names match REGEXP."
 	      (push (expand-file-name file dir) files)))))
       (nconc result (nreverse files)))))
 
+;; `string-collate-lessp' is new in Emacs 25.
+(defalias 'org-string-collate-lessp
+  (if (fboundp 'string-collate-lessp)
+      'string-collate-lessp
+    'string-lessp))
+
 
 ;;; Obsolete aliases (remove them after the next major release).
 

lisp/org.el

@@ -3558,8 +3558,8 @@ is better to limit inheritance to certain tags using the variables
   :group 'org-tags
   :type '(choice
 	  (const :tag "No sorting" nil)
-	  (const :tag "Alphabetical" string<)
-	  (const :tag "Reverse alphabetical" string>)
+	  (const :tag "Alphabetical" org-string-collate-lessp)
+	  (const :tag "Reverse alphabetical" org-string-collate-greaterp)
 	  (function :tag "Custom function" nil)))
 
 (defvar org-tags-history nil
@@ -8791,7 +8791,7 @@ function is being called interactively."
 	     (t (error "Invalid sorting type `%c'" sorting-type))))
 	  nil
 	  (cond
-	   ((= dcst ?a) 'string<)
+	   ((= dcst ?a) 'org-string-collate-lessp)
 	   ((= dcst ?f)
 	    (or compare-func
 		(and interactive?
@@ -8901,6 +8901,12 @@ Possible values in the list of contexts are `table', `headline', and `item'."
 		    (org-in-item-p)))
       (goto-char pos))))
 
+;; Defined to provide a value for defcustom, since there is no
+;; string-collate-greaterp in Emacs.
+(defun org-string-collate-greaterp (s1 s2)
+  "Return non-nil if S1 is greater than S2 in collation order."
+  (not (org-string-collate-lessp s1 s2)))
+
 ;;;###autoload
 (defun org-run-like-in-org-mode (cmd)
   "Run a command, pretending that the current buffer is in Org mode.

testing/lisp/test-org.el

@@ -2745,6 +2745,23 @@ http://article.gmane.org/gmane.emacs.orgmode/21459/"
 	  (org-test-with-temp-text "\n* def\n* xyz\n* abc\n"
 	    (org-sort-entries nil ?A)
 	    (buffer-string))))
+  ;; Sort alphabetically (with non-ASCII input).  Rebinds
+  ;; `string-collate-lessp' to enforce a canonical locale during
+  ;; testing.
+  (let ((original-string-collate-lessp (symbol-function 'string-collate-lessp)))
+    (cl-letf (((symbol-function 'string-collate-lessp)
+	       (lambda (s1 s2)
+		 (funcall original-string-collate-lessp s1 s2 "C"))))
+      (should
+       (equal "\n* ¥\n* §\n"
+	      (org-test-with-temp-text "\n* §\n* ¥"
+	        (org-sort-entries nil ?a)
+	        (buffer-string))))
+      (should
+       (equal "\n* §\n* ¥\n"
+	      (org-test-with-temp-text "\n* §\n* ¥"
+	        (org-sort-entries nil ?A)
+	        (buffer-string))))))
   ;; Sort numerically.
   (should
    (equal "\n* 1\n* 2\n* 10\n"
@@ -2935,6 +2952,11 @@ SCHEDULED: <2017-05-06 Sat>
 	    (org-sort-entries nil ?a)
 	    (buffer-string)))))
 
+(ert-deftest test-org/string-collate-greaterp ()
+  "Test `org-string-collate-greaterp' specifications."
+  (should (org-string-collate-greaterp "def" "abc"))
+  (should-not (org-string-collate-greaterp "abc" "def")))
+
 (ert-deftest test-org/file-contents ()
   "Test `org-file-contents' specifications."
   ;; Open files.