bug#41706: 26.1; sort-subr predicate cannot be set successfully

["Marvin Gülker" <post+ebugs@guelker.eu>]

Dear maintainers,

I was trying to define a variant of `sort-lines' which sorts lines 
according to the current locale's rules, i.e. instead of comparing the 
strings with the default (as the docs of the underlying function 
`sort-subr' say) predicate of `string<' they should be compared with 
`string-collate-lessp'. I copied the source code of `sort-lines' and 
adapted the call to `sort-subr' to use `string-collate-lessp', resulting 
in this function:

(defun my-sort-lines-collate (reverse beg end)
  (interactive "P\nr")
  (save-excursion
    (save-restriction
      (narrow-to-region beg end)
      (goto-char (point-min))
      (let ;; To make `end-of-line' and etc. to ignore fields.
	  ((inhibit-field-text-motion t))
	(sort-subr reverse 'forward-line 'end-of-line nil nil 'string-collate-lessp)))))

This function, when called, only results in an error "Wrong type 
argument: stringp" (tested in `emacs -Q'). Interestingly, if the 6th 
argument to `sort-subr' is changed to `string<' (which is said to be the 
default value!) the very same error results.

It does not make a difference to explicitely define it with a lambda 
instead:

(defun my-sort-lines-collate (reverse beg end)
  (interactive "P\nr")
  (save-excursion
    (save-restriction
      (narrow-to-region beg end)
      (goto-char (point-min))
      (let ;; To make `end-of-line' and etc. to ignore fields.
	  ((inhibit-field-text-motion t))
	(sort-subr reverse 'forward-line 'end-of-line nil nil (lambda (a b) (string-collate-lessp a b)))))))

This code results in the same error.

I assume this is a kind of bug. Either way, I'd like to request as a 
feature a variant of `sort-lines' that is aware of the locale's sorting 
rules.


In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.5)
 of 2019-09-23, modified by Debian built on x86-grnet-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description:	Debian GNU/Linux 10 (buster)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set
my-sort-lines-collate
Mark set
Wrong type argument: stringp, (193 . 215)
Configured using:
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