[PATCH v2 2/5] libstdc++: Support ON-format DAY in Zone UNTIL field [PR124852]
Jonathan Wakely
jwakely@redhat.com
Tue May 12 09:01:07 GMT 2026
On Sun, 26 Apr 2026 at 19:53 -0400, Álvaro Begué wrote:
>The Zone-line UNTIL parser only accepted a plain day-of-month integer
>for the DAY field, while the tzdata.zi grammar accepts the same ON-style
>forms as Rule lines: lastSun, Sun>=8, Sat<=20, etc. Real zones use these
>forms in their UNTIL DAY: Europe/Simferopol's `3 - MSK 1997 Mar lastSu
>1u`, for instance, became `Mar 1` (silently misparsed) instead of `Mar
>30`, leaving Simferopol an extra 29 days in MSK.
>
>The previous parser's `int d = 1; in >> m >> d >> t;` chain silently
>left d == 1 when the day token wasn't a digit, then went on to parse the
>remainder as the TIME field.
>
>Factor out the day-component parser from operator>>(istream&, on_day&)
>as parse_day_spec(), and reuse it for the UNTIL DAY field. parse_day_spec
>handles all three on_day forms (DayOfMonth, LastWeekday, LessEq /
>GreaterEq). The MONTH-only and YEAR-only short forms are still accepted
>because the DAY/TIME fields are optional and default to day 1, time 00:00.
>The on_day struct's pin() method handles the year/month-relative
>resolution.
>
>The DAY field is unambiguously distinguishable from a TIME field that
>could otherwise follow the MONTH directly: per zic's grammar, MONTH
>must be followed by DAY before any TIME is allowed. So we always
>attempt to parse a DAY if any non-whitespace remains after the MONTH.
>
>libstdc++-v3/ChangeLog:
>
>PR libstdc++/124852
>* src/c++20/tzdb.cc (parse_day_spec): New function, factored
>out of operator>>(istream&, on_day&).
>(operator>>(istream&, on_day&)): Use parse_day_spec.
>(operator>>(istream&, ZoneInfo&)): Replace the integer DAY
>parser with parse_day_spec for the UNTIL field.
>* testsuite/std/time/time_zone/until_day_on.cc: New test.
>
>Signed-off-by: Álvaro Begué <alvaro.begue@gmail.com>
>---
> libstdc++-v3/src/c++20/tzdb.cc | 47 +++--
> .../std/time/time_zone/until_day_on.cc | 168 ++++++++++++++++++
> 2 files changed, 199 insertions(+), 16 deletions(-)
> create mode 100644
>libstdc++-v3/testsuite/std/time/time_zone/until_day_on.cc
>
>diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
>index 1e49bb749..c0d62bc35 100644
>--- a/libstdc++-v3/src/c++20/tzdb.cc
>+++ b/libstdc++-v3/src/c++20/tzdb.cc
>@@ -2200,22 +2200,23 @@ namespace std::chrono
> }
> };
>
>- istream& operator>>(istream& in, on_day& to)
>+ // Read the day-component of an on_day expression (everything after the
>+ // month). Three forms are accepted: a plain day-of-month number,
>+ // "lastXxx" where Xxx is a weekday name (LastWeekday), or "Xxx<=N" or
>+ // "Xxx>=N" (LessEq / GreaterEq). On failure the function sets failbit
>+ // and leaves `on` unchanged.
>+ istream&
>+ parse_day_spec(istream& in, on_day& on)
This makes me regret naming the struct 'on_day' when it's not just the
ON field, it's the IN and ON fields combined. If I'd named that with
a better name, we could have both:
operator>>(istream&, in_on_day&);
and
operator>>(istream&, on_day&);
instead of using operator>> for most types, and then having a named
'parse_day_spec' function for just one thing.
We can do this:
diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index 587e88bc0dd5..5ad865a3e760 100644
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -372,6 +372,14 @@ namespace std::chrono
}
friend istream& operator>>(istream&, on_day&);
+
+ struct day_t // tag type for reading ON and DAY fields only
+ {
+ on_day* parent;
+ friend istream& operator>>(istream&, day_t&&);
+ };
+
+ day_t day() { return day_t{this}; }
};
// Wrapper for two chrono::year values, which reads the FROM and TO
And then the parse_day_spec function becomes the operator>> for
on_day::day_t, and you would use it like this:
+ istream& operator>>(istream& in, on_day& to)
+ {
+ on_day on{};
+ abbrev_month m{};
+ in >> m;
+ on.month = static_cast<unsigned>(m.m);
+ if (in >> on.day())
+ to = on;
+ return in;
+ }
I would prefer to consistently use operator>> for parsing, instead of
a mixture of operator>> and named functions.
> {
>- on_day on{};
>- abbrev_month m{};
>- in >> m;
>- on.month = static_cast<unsigned>(m.m);
> int c = ws(in).peek();
> if ('0' <= c && c <= '9')
> {
>- on.kind = on_day::DayOfMonth;
All the leading TABs have been stripped from this patch again :-(
This makes it hard to review and even harder to apply the patch.
If you can't use git-send-email to send the patch, please attach a
patch.txt file in Gmail, which prevents it from being munged by Gmail.
> unsigned d;
> in >> d;
> if (d <= 31) [[likely]]
> {
>+ on.kind = on_day::DayOfMonth;
> on.day_of_month = d;
>- to = on;
> return in;
> }
> }
>@@ -2226,7 +2227,6 @@ namespace std::chrono
> {
> on.kind = on_day::LastWeekday;
> on.day_of_week = w.wd.c_encoding();
>- to = on;
> return in;
> }
> }
>@@ -2238,14 +2238,13 @@ namespace std::chrono
> {
> if (in.get() == '=')
> {
>- on.kind = c == '<' ? on_day::LessEq : on_day::GreaterEq;
>- on.day_of_week = w.wd.c_encoding();
> unsigned d;
> in >> d;
> if (d <= 31) [[likely]]
> {
>+ on.kind = c == '<' ? on_day::LessEq : on_day::GreaterEq;
>+ on.day_of_week = w.wd.c_encoding();
> on.day_of_month = d;
>- to = on;
> return in;
> }
> }
>@@ -2255,6 +2254,17 @@ namespace std::chrono
> return in;
> }
>
>+ istream& operator>>(istream& in, on_day& to)
>+ {
>+ on_day on{};
>+ abbrev_month m{};
>+ in >> m;
>+ on.month = static_cast<unsigned>(m.m);
>+ if (parse_day_spec(in, on))
>+ to = on;
>+ return in;
>+ }
>+
> istream& operator>>(istream& in, at_time& at)
> {
> int sign = 1;
>@@ -2357,12 +2367,17 @@ namespace std::chrono
> in.exceptions(ios::goodbit); // Don't throw ios::failure if YEAR
>absent.
> if (int y = int(year::max()); in >> y)
> {
>- abbrev_month m{January};
>- int d = 1;
>+ on_day on{.kind = on_day::DayOfMonth, .month = 1, .day_of_month = 1};
> at_time t{};
>- // XXX DAY should support ON format, e.g. lastSun or Sun>=8
>- in >> m >> d >> t;
>- inf.m_until = sys_days(year(y)/m.m/day(d)) + seconds(t.time);
>+ if (abbrev_month m{January}; in >> m)
>+ {
>+ on.month = static_cast<unsigned>(m.m);
>+ if (!ws(in).eof())
>+ if (parse_day_spec(in, on))
>+ in >> t;
This could be:
if (in >> on.day() >> t)
You could have written it like that already, because parse_day_spec
returns the stream, so:
if (parse_day_spec(in, on) >> t)
but I think it's a lot more clear what's happening when written as two
chained operator>> calls.
There's not much benefit to returning istream& from parse_day_spec
because your patch only ever uses it as a bool anyway. But if we
replace it with another operator>> overload then it makes sense to use
it in chained expressions like this.
>+ }
>+ year_month_day ymd = on.pin(year(y));
>+ inf.m_until = sys_days(ymd) + seconds(t.time);
> if (t.indicator != at_time::Universal)
> { // UNTIL uses "the rules in effect just before the transition"
> // adjust by STDOFF
>diff --git a/libstdc++-v3/testsuite/std/time/time_zone/until_day_on.cc
>b/libstdc++-v3/testsuite/std/time/time_zone/until_day_on.cc
>new file mode 100644
>index 000000000..6b5ce6bae
>--- /dev/null
>+++ b/libstdc++-v3/testsuite/std/time/time_zone/until_day_on.cc
>@@ -0,0 +1,168 @@
>+// { dg-do run { target c++20 } }
>+// { dg-require-effective-target tzdb }
>+// { dg-require-effective-target cxx11_abi }
>+// { dg-xfail-run-if "no weak override on AIX" { powerpc-ibm-aix* } }
>+
>+// The DAY portion of a Zone line's UNTIL field accepts not only a
>+// numeric day-of-month but also "lastXxx" (last weekday in the month)
>+// and "Xxx<=N" / "Xxx>=N" forms, just like the ON field of a Rule line.
>+//
>+// Real-world example: Europe/Simferopol has
>+// 3 - MSK 1997 Mar lastSu 1u
>+// which places the boundary on 1997-03-30 (the last Sunday of March).
>+
>+#include <chrono>
>+#include <fstream>
>+#include <testsuite_hooks.h>
>+
>+static bool override_used = false;
>+
>+namespace __gnu_cxx
>+{
>+ const char* zoneinfo_dir_override() {
>+ override_used = true;
>+ return "./";
>+ }
>+}
>+
>+void
>+test_lastsu()
>+{
>+ using namespace std::chrono;
>+
>+ std::ofstream("tzdata.zi") << R"(# version test_lastsu
>+Z Test/LastSu 3 - MSK 1997 Mar lastSu 1u
>+ 3 - X
>+)";
>+
>+ const auto& db = reload_tzdb();
>+ VERIFY( override_used );
There should be the same XFAIL comment on this line above.
>+ VERIFY( db.version == "test_lastsu" );
>+
>+ auto* tz = locate_zone("Test/LastSu");
>+
>+ // True boundary: 1997-03-30 01:00 UTC (lastSu of March 1997 is Mar 30,
>+ // and the indicator is 'u' = Universal so no offset adjustment).
>+ sys_seconds boundary = sys_days{1997y/March/30} + 1h;
>+
>+ // Just before: still in the MSK line.
>+ auto before = tz->get_info(boundary - 1s);
>+ VERIFY( before.abbrev == "MSK" );
>+ VERIFY( before.offset == 3h );
>+
>+ // At/after the boundary: in the X line.
>+ auto at = tz->get_info(boundary);
>+ VERIFY( at.abbrev == "X" );
>+
>+ // Check that the lastSu day is parsed correctly, and not defaulted
>+ // to the 1st: a March 15 query must still be in the MSK line.
>+ auto mid_march = tz->get_info(sys_days{1997y/March/15});
>+ VERIFY( mid_march.abbrev == "MSK" );
>+ VERIFY( mid_march.offset == 3h );
>+}
>+
>+void
>+test_sun_ge_n()
>+{
>+ using namespace std::chrono;
>+
>+ std::ofstream("tzdata.zi") << R"(# version test_sun_ge_n
>+Z Test/SunGE 0 - A 1990 Jun Sun>=8 0u
>+ 0 - B
>+)";
>+
>+ const auto& db = reload_tzdb();
>+ VERIFY( override_used );
>+ VERIFY( db.version == "test_sun_ge_n" );
>+
>+ auto* tz = locate_zone("Test/SunGE");
>+
>+ // First Sunday >= June 8 1990 = June 10 (June 8 1990 was a Friday).
>+ sys_seconds boundary = sys_days{1990y/June/10};
>+
>+ auto before = tz->get_info(boundary - 1s);
>+ VERIFY( before.abbrev == "A" );
>+ auto at = tz->get_info(boundary);
>+ VERIFY( at.abbrev == "B" );
>+
>+ // Check that Sun>=8 is parsed correctly, and not defaulted to the 1st.
>+ auto early = tz->get_info(sys_days{1990y/June/1});
>+ VERIFY( early.abbrev == "A" );
>+}
>+
>+void
>+test_sun_le_n()
>+{
>+ using namespace std::chrono;
>+
>+ std::ofstream("tzdata.zi") << R"(# version test_sun_le_n
>+Z Test/SunLE 0 - A 1990 Jun Sun<=15 0u
>+ 0 - B
>+)";
>+
>+ const auto& db = reload_tzdb();
>+ VERIFY( override_used );
>+ VERIFY( db.version == "test_sun_le_n" );
>+
>+ auto* tz = locate_zone("Test/SunLE");
>+
>+ // Last Sunday <= June 15 1990 = June 10.
>+ sys_seconds boundary = sys_days{1990y/June/10};
>+
>+ auto before = tz->get_info(boundary - 1s);
>+ VERIFY( before.abbrev == "A" );
>+ auto at = tz->get_info(boundary);
>+ VERIFY( at.abbrev == "B" );
>+}
>+
>+void
>+test_year_only()
>+{
>+ using namespace std::chrono;
>+
>+ // MONTH, DAY and TIME default to January 1st 00:00 if not specified.
>+ std::ofstream("tzdata.zi") << R"(# version test_year_only
>+Z Test/YearOnly 0 - A 1990
>+ 0 - B
>+)";
>+
>+ const auto& db = reload_tzdb();
>+ VERIFY( db.version == "test_year_only" );
>+
>+ auto* tz = locate_zone("Test/YearOnly");
>+ auto before = tz->get_info(sys_days{1989y/December/31} + 23h);
>+ VERIFY( before.abbrev == "A" );
>+ auto at = tz->get_info(sys_days{1990y/January/1});
>+ VERIFY( at.abbrev == "B" );
>+}
>+
>+void
>+test_year_month_only()
>+{
>+ using namespace std::chrono;
>+
>+ // DAY and TIME default to the 1st 00:00 if not specified.
>+ std::ofstream("tzdata.zi") << R"(# version test_year_month_only
>+Z Test/YearMonth 0 - A 1990 Jul
>+ 0 - B
>+)";
>+
>+ const auto& db = reload_tzdb();
>+ VERIFY( db.version == "test_year_month_only" );
>+
>+ auto* tz = locate_zone("Test/YearMonth");
>+ auto before = tz->get_info(sys_days{1990y/June/30} + 23h);
>+ VERIFY( before.abbrev == "A" );
>+ auto at = tz->get_info(sys_days{1990y/July/1});
>+ VERIFY( at.abbrev == "B" );
>+}
>+
>+int
>+main()
>+{
>+ test_lastsu();
>+ test_sun_ge_n();
>+ test_sun_le_n();
>+ test_year_only();
>+ test_year_month_only();
>+}
>--
>2.34.1
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