On Thu, 16 Jul 2026 at 12:06, Tomasz Kamiński <[email protected]> wrote:
>
> When seeding zone information from the application of the rules
> prior the transition, we should consider the rule triggering exactly
> at boundary, not only prior, i.e. call find_active_rule with info.begin.
> Due the rule_start - t < days(1) check (that is not removed yet), such
> transitions were effectively ignored.
>
> This addresses the regression introduced by r17-2060-gf7cde200320e08,
> where new differences between libstdc++ output and date library where
> introduced fro some zones (e.g. Europe/Lisbon in 1976). With this fixup,
> the patch is now pure improvement in consistency.
>
> libstdc++-v3/ChangeLog:
>
> * src/c++20/tzdb.cc (time_zone::_M_get_sys_info): Find
> rules firing exactly at transition time.
> * testsuite/std/time/time_zone/wall_cascade.cc: Add test
> for Europe/Lisbon, and corrected test_negative.
> ---
> Using your test program, I compared current trunk state with GCC-16
> and found that we have new differences caused by my previous patch.
> All of them are now addressed. Note that this would most likely be
> fixed in follow-up patches, but I think that r17-2060-gf7cde200320e08,
> with this fix may be good candidate for backport, as they should not
> impact performance.
>
> Testing on x86_64-linux locally. *time_zone* test passed with all
> standard modes. OK for trunk?
OK with one typo fixed ("consideration")
>
> libstdc++-v3/src/c++20/tzdb.cc | 2 +-
> .../std/time/time_zone/wall_cascade.cc | 52 ++++++++++++++++---
> 2 files changed, 47 insertions(+), 7 deletions(-)
>
> diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
> index 998a363151d..4ecce9e214e 100644
> --- a/libstdc++-v3/src/c++20/tzdb.cc
> +++ b/libstdc++-v3/src/c++20/tzdb.cc
> @@ -1027,7 +1027,7 @@ namespace std::chrono
> // SAVE and LETTERS values. There may not be a Rule for the period
> // before the first DST transition, so find the earliest DST->STD
> // transition and use the LETTERS from that.
> - if (const Rule* active_rule = find_active_rule(rules, info.begin -
> seconds(1), ri.offset()))
> + if (const Rule* active_rule = find_active_rule(rules, info.begin,
> ri.offset()))
> {
> info.offset = ri.offset() + active_rule->save;
> info.save = chrono::duration_cast<minutes>(active_rule->save);
> diff --git a/libstdc++-v3/testsuite/std/time/time_zone/wall_cascade.cc
> b/libstdc++-v3/testsuite/std/time/time_zone/wall_cascade.cc
> index 2707e3fcd33..f270a13fcad 100644
> --- a/libstdc++-v3/testsuite/std/time/time_zone/wall_cascade.cc
> +++ b/libstdc++-v3/testsuite/std/time/time_zone/wall_cascade.cc
> @@ -76,11 +76,11 @@ test_negative()
> // running save.
> //
> // Two-line zone whose second line begins at 1945 Sep 16 01:00 UT,
> - // at the cascaded firing time (Sep 16 01:00 UT), but after
> + // before the cascaded firing time (Sep 16 02:00 UT), but after
> // non-cascaded firing time (Sep 16 00:00 UT) of the September rule.
> // The seeding must pick the April rule (save=-2, CEST) at info.begin.
> std::ofstream("tzdata.zi") << R"(# version test_negative_cascade
> -R Fr 1945 o - Apr 2 2 -2 M
> +R Fr 1945 o - Apr 2 2 -3 M
> R Fr 1945 o - Sep 16 0 0 -
> Z Test/Negative 0 - X 1945 Sep 16 1u
> 1 Fr CE%sT
> @@ -96,15 +96,24 @@ Z Test/Negative 0 - X 1945 Sep 16 1u
> // one second after.
> auto info = tz->get_info(sys_seconds{
> sys_days(1945y/September/16) + 1h + 1s});
> - VERIFY( info.offset == -1h );
> - VERIFY( info.save == -2h );
> + VERIFY( info.offset == -2h );
> + VERIFY( info.save == -3h );
> VERIFY( info.abbrev == "CEMT" );
>
> // The boundary instant.
> auto at_boundary
> = tz->get_info(sys_seconds{sys_days(1945y/September/16) + 1h});
> - VERIFY( at_boundary.offset == -1h );
> - VERIFY( at_boundary.save == -2h );
> + VERIFY( at_boundary.offset == -2h );
> + VERIFY( at_boundary.save == -3h );
> +
> + // Test the firing of Sep 16 rule
> + auto at_sep_rule = tz->get_info(sys_seconds{
> + sys_days(1945y/September/16) + 2h});
> + // The transition_window < 1d condition triggers, and
> + // transition is ignored.
> + // VERIFY( at_sep_rule.offset == 1h );
> + // VERIFY( at_sep_rule.save == 0h );
> + // VERIFY( at_sep_rule.abbrev == "CET" );
> }
>
> void
> @@ -222,6 +231,36 @@ Z Test/EarielYear 11:39:4 - LMT 1868 N 2
> VERIFY( at_boundary.abbrev == "EYMT" );
> }
>
> +void
> +test_at_boundary()
> +{
> + std::ofstream("tzdata.zi") << R"(# version test_at_boundary
> +R p 1947 1966 - Ap Su>=1 2s 1 S
> +R p 1947 1965 - O Su>=1 2s 0 -
> +R p 1976 o - S lastSu 1 0 -
> +R p 1977 o - Mar lastSu 0s 1 S
> +R p 1977 o - S lastSu 0s 0 -
> +Z Europe/Lisbon -0:36:45 - LMT 1884
> +1 - CET 1976 S 26 1
> +0 p WE%sT 1986
> + )";
> +
> + const auto& db = reload_tzdb();
> + VERIFY( override_used ); // If this fails then XFAIL for the target.
> + VERIFY( db.version == "test_at_boundary" );
> +
> + // The change from CET to WE%sT line happens 1976 Sep 26 00:00:00 UT,
> + // should take into considration 1976 lastSu rule that fires at the
> + // same time (running save is 1h from 1966 Ap rule application),
> + // and start in standard time (WET period).
> + auto* utz = locate_zone("Europe/Lisbon");
> + auto at_boundary
> + = utz->get_info(sys_seconds{sys_days(1976y/September/26) + 0h});
> + VERIFY( at_boundary.offset == 0h );
> + VERIFY( at_boundary.save == 0min );
> + VERIFY( at_boundary.abbrev == "WET" );
> +}
> +
> int
> main()
> {
> @@ -230,4 +269,5 @@ main()
> test_next_year();
> test_prev_year();
> test_earlier_year();
> + test_at_boundary();
> }
> --
> 2.55.0
>
