bug#55635: `make-decoded-time' incorrectly sets DST to nil, it should be -1 (guess)

[Maxim Nikulin]

On 14/06/2022 04:30, Paul Eggert wrote:
> On 5/31/22 05:25, Maxim Nikulin wrote:
>
> > I have never used `cl-defstruct' before (and frankly speaking I am 
> > rather confused that the `decoded-time' struct and its constructor 
> > `make-decoded-time' are defined in different files and even 
> > directories are not the same), so my question may be naïve. Why did 
> > not you just add this new sentence to the :documentation property of 
> > the DST slot a bit above?
>
> I am not sure I understand the question. The slot itself has a specified 
> value (t, nil, or -1) whereas the decoded-time-dst function returns an 
> unspecified value when there is no slot.

See the diff at the end of this message. Maybe I do not see any 
difference because I tried such definitions and (describe-function 
`decoded-time-dst), (describe-symbol 'decoded-time) in Emacs-27, not for 
the development version.

> > It may be reasonable to cross-link `decoded-time' and 
> > `make-decoded-time' in docstrings.
>
> Thanks for the suggestions; I installed the attached.

First of all, I am sorry, for some reason I missed that 
`make-decoded-time' already has "`decoded-time'" in its docstring. Maybe 
I am not familiar with help system enough, but my idea was to add a 
docstring to `cl-defstruct' as a whole (currently only slots are 
documented) that has "`make-decoded-time'" reference in the opposite 
direction to existing one. I admit that I missed something else and it 
is not necessary.

diff --git a/lisp/simple.el b/lisp/simple.el
index 99c951b24b..3054d79d44 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -10511,27 +10511,19 @@ This is an integer between 1 and 12 
(inclusive).  January is 1.")
   (year nil :documentation "This is a four digit integer.")
   (weekday nil :documentation "\
 This is a number between 0 and 6, and 0 is Sunday.")
+  ;; decoded-time-dst is problematic on 6-element lists.
+  ;; It should return -1 indicating unknown DST, but currently returns
+  ;; nil indicating standard time.
   (dst -1 :documentation "\
 This is t if daylight saving time is in effect, nil if it is not
 in effect, and -1 if daylight saving information is not available.
-Also see `decoded-time-dst'.")
+As a special case, return an unspecified value when given a list
+too short to have a dst element.")
   (zone nil :documentation "\
 This is an integer indicating the UTC offset in seconds, i.e.,
 the number of seconds east of Greenwich.")
   )

-;; Document that decoded-time-dst is problematic on 6-element lists.
-;; It should return -1 indicating unknown DST, but currently returns
-;; nil indicating standard time.
-(put 'decoded-time-dst 'function-documentation
-     "Access slot \"dst\" of `decoded-time' struct CL-X.
-This is t if daylight saving time is in effect, nil if it is not
-in effect, and -1 if daylight saving information is not available.
-As a special case, return an unspecified value when given a list
-too short to have a dst element.
-
-(fn CL-X)")
-
 (defun get-scratch-buffer-create ()
   "Return the *scratch* buffer, creating a new one if needed."
   (or (get-buffer "*scratch*")