At indicated by the pre-existing comment, the lazy zone expansion can be
only resumed from STD (save == 0) zone. However, the current condition
for stopping on DST (save != 0) doesn't ensure that, as some rule specify
transitions between DST zones. For example Y rule used by America/Dawson
August 1945 transion only change letters:
Y 1942 o - F 9 2 1 W
Y 1945 o - Au 14 23u 1 P
This patch correct the condition, by using next_rule (i.e. one applying
after last expanded zone): either there is no zone (we default to STD)
or it have save zero.
libstdc++-v3/ChangeLog:
PR libstdc++/116110
* src/c++20/tzdb.cc (time_zone::_M_get_sys_info): Correct
condition for stopping zone geration before STD zone.
* testsuite/std/time/time_zone/116110.cc (test_dawson):
Add test for America/Dawson August 1945 transition.
---
This addresses the root cause and provides much more targeted solution
for the problem described "Always seed info.offset and info.save from
the active rule." in Alvaro 5/5 patch:
https://gcc.gnu.org/pipermail/libstdc++/2026-May/066526.html.
As this change is very minimal and affect only cases of sequence of
DST zones (we expand more zones), I think this is very safe to backport.
Testing on x86_64-linux. *time_zone* test passed, in all standard modes
and -m32/debug. OK for trunk and GCC-16.
libstdc++-v3/src/c++20/tzdb.cc | 4 +-
.../testsuite/std/time/time_zone/116110.cc | 41 +++++++++++++++++++
2 files changed, 43 insertions(+), 2 deletions(-)
diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index 460a242fed1..99b3ed58d95 100644
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -1231,10 +1231,10 @@ namespace std::chrono
result_index = new_infos.size() - 1;
else if (result_index >= 0 && !merged)
{
- // Finish on a DST sys_info if possible, so that if we resume
+ // Finish before a STD sys_info if possible, so that if we resume
// generating sys_info objects after this time point, save=0
// should be correct for the next sys_info.
- if (num_after > 1 || info.save != 0min)
+ if (num_after > 1 || !next_rule || next_rule->save == 0s)
--num_after;
}
diff --git a/libstdc++-v3/testsuite/std/time/time_zone/116110.cc
b/libstdc++-v3/testsuite/std/time/time_zone/116110.cc
index 1ec01d35955..45daed8c050 100644
--- a/libstdc++-v3/testsuite/std/time/time_zone/116110.cc
+++ b/libstdc++-v3/testsuite/std/time/time_zone/116110.cc
@@ -78,9 +78,50 @@ test_apia()
VERIFY( info.abbrev == "+14" );
}
+void
+test_dawson()
+{
+ /* 1945 August rule change changes letters (abbrev)
+ and remains in DST:
+ R Y 1942 o - F 9 2 1 W
+ R Y 1945 o - Au 14 23u 1 P
+ R Y 1945 o - S 30 2 0 S
+ Z America/Dawson -9:17:40 - LMT 1900 Au 20
+ -9 Y Y%sT 1965
+ -9 Yu Y%sT 1973 O 28
+ */
+
+ auto* tz = locate_zone("America/Dawson");
+
+ // Triggers rule transitions from the start.
+ sys_info info = tz->get_info(sys_days(1900y/August/20) + 10h);
+ VERIFY( info.offset == -9h );
+ VERIFY( info.save == 0min );
+ VERIFY( info.abbrev == "YST" );
+
+ // Check YWT transition at 02:00 + 9h UT, that is DST
+ info = tz->get_info(sys_days(1942y/February/9) + 11h);
+ VERIFY( info.offset == -8h );
+ VERIFY( info.save == 60min );
+ VERIFY( info.abbrev == "YWT" );
+
+ // Check YPT transition at 23:00 UT, remains DST
+ info = tz->get_info(sys_days(1945y/August/14) + 23h);
+ VERIFY( info.offset == -8h );
+ VERIFY( info.save == 60min );
+ VERIFY( info.abbrev == "YPT" );
+
+ // Check YST transition at 02:00 + 8h UT, switches to STD
+ info = tz->get_info(sys_days(1945y/September/30) + 10h);
+ VERIFY( info.offset == -9h );
+ VERIFY( info.save == 0min );
+ VERIFY( info.abbrev == "YST" );
+}
+
int main()
{
test_broken_hill();
test_kiritimati();
test_apia();
+ test_dawson();
}
--
2.55.0
