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

[Paul Eggert <eggert@cs.ucla.edu>]

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On 5/27/22 03:40, Lars Ingebrigtsen wrote:
>> This looks wrong. Shouldn't it leave the DST flag alone? I.e., just this:
>>
>>    (unless (decoded-time-zone time)
>>      (setf (decoded-time-zone-time) default-zone))
>>
>> That is, if we assume that for the DST component -1 means "unknown"
>> and nil means "standard time", it should be OK for
>> decoded-time-set-defaults to leave the DST component alone, for the
>> same reason that it leaves the DOW component alone.

> Yes, I think so.  But you changed this in a391ffa2f03, and you usually
> have a good reason for changes like this, so I thought there must be
> something subtle going on here I didn't quite get.  😀

Thanks for the compliment, not sure it's deserved here....


> The old code doesn't look quite right, either, I think...
> 
> -  ;; When we don't have a time zone and we don't have a DST, then mark
> -  ;; it as unknown.
> -  (when (and (not (decoded-time-zone time))
> -             (not (decoded-time-dst time)))
> -    (setf (decoded-time-dst time) -1))
> -
> -  (when (and (not (decoded-time-zone time))
> -             default-zone)
> -    (setf (decoded-time-zone time) 0))

Yes, that old code was wrong because it incorrectly assumeed that (not 
(decoded-time-dst time)) means the DST flag is unspecified, whereas it 
really means that the DST flag is specifying standard time.

It also looked odd because default-zone was used only as a boolean, even 
though its name suggests that it's the default time zone. This usage 
dates back to commit fa648a59c9818ae284209ac7ae4f3700aebd92c9 which you 
installed in July 2019. The only call using default-zone in Emacs is in 
newsticker--decode-iso8601-date, which passes 0 so that the oddity in 
the implementation makes no difference there.

Part of the confusion here is that nil doesn't mean "no time zone is 
known"; it means "use the Emacs default time zone". In other words, nil 
has the same interpretation problem in time zones that it has in DST 
flags - it doesn't mean "unknown".

To try to lessen the confusion I installed the attached, which fixes the 
reported bug so I'll close the bug report. Please feel free to revert if 
you see a problem with it (I'm just trying to save time here by being bold).

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From fe38cbc14fb627f39e145b9a85f029af96f903c7 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Fri, 27 May 2022 12:19:43 -0700
Subject: [PATCH] decoded-time-set-defaults now leaves DST alone
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

* lisp/calendar/time-date.el (decoded-time-set-defaults):
Don’t mess with decoded-time-dst (Bug#55635).
---
 lisp/calendar/time-date.el | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/lisp/calendar/time-date.el b/lisp/calendar/time-date.el
index dc77a7c7e0..40374c3bb4 100644
--- a/lisp/calendar/time-date.el
+++ b/lisp/calendar/time-date.el
@@ -562,6 +562,9 @@ decoded-time-set-defaults
 This year is used to guarantee portability; see Info
 node `(elisp) Time of Day'.
 
+Optional argument DEFAULT-ZONE specifies what time zone to
+default to when TIME's time zone is nil (meaning local time).
+
 TIME is modified and returned."
   (unless (decoded-time-second time)
     (setf (decoded-time-second time) 0))
@@ -577,13 +580,11 @@ decoded-time-set-defaults
   (unless (decoded-time-year time)
     (setf (decoded-time-year time) 1970))
 
-  ;; When we don't have a time zone, default to DEFAULT-ZONE without
-  ;; DST if DEFAULT-ZONE if given, and to unknown DST otherwise.
   (unless (decoded-time-zone time)
-    (if default-zone
-	(progn (setf (decoded-time-zone time) default-zone)
-	       (setf (decoded-time-dst time) nil))
-      (setf (decoded-time-dst time) -1)))
+    (setf (decoded-time-zone time) default-zone))
+
+  ;; Do not set decoded-time-weekday or decoded-time-dst,
+  ;; as encode-time can infer them well enough when unknown.
 
   time)
 
-- 
2.34.1