[PATCH v2 1/5] libstdc++: Fix numeric save offset on Zone lines [PR124851]

[Tomasz Kaminski]

On Mon, Apr 27, 2026 at 1:51 AM Álvaro Begué <alvaro.begue@gmail.com> wrote:

> When a Zone line specifies a numeric value as its RULES field (the
> constant DST save value for that zone line, e.g. Africa/Gaborone's
> "2 1 CAST" line), the parser stored the standard offset alone in
> ZoneInfo::m_offset. ZoneInfo::to() then returned that as
> sys_info::offset, dropping the numeric save and reporting a total
> offset that was wrong by the save amount.
>
> This was inconsistent with the two ZoneInfo constructors that take a
> sys_info, which previously stored the *total* offset (stdoff + save) in
> m_offset. As a result m_offset's semantics depended on which code path
> created the ZoneInfo, and only the parser path's lines with non-zero
> numeric save were observably broken.
>
> Fix by giving m_offset a single semantics: always the standard offset
> only. The two sys_info-taking constructors now subtract the save before
> storing, and to() adds it back when reconstructing the sys_info.
>
> The remaining .offset() callers inside _M_get_sys_info already expect
> the standard offset (they are computing rule firing times, where the
> save component is added separately from the active rule's save value),
> so no other call sites need adjustment.
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/124851
> * src/c++20/tzdb.cc (ZoneInfo::ZoneInfo(sys_info&&)): Store
> stdoff only in m_offset (subtract info.save).
> (ZoneInfo::ZoneInfo(const pair<sys_info, string_view>&)):
> Likewise.
> (ZoneInfo::offset()): Document new semantics.
> (ZoneInfo::to(sys_info&)): Add m_save back to offset() when
> populating sys_info::offset.
> * testsuite/std/time/time_zone/numeric_save.cc: New test.
>
It seems that all tabs at the begining of the line were removed by your
mail program,
this made it very curbersome to apply the patch, but I reproduced it
locally.

The changes  LGTM. I will push it to trunk after it gets approved.

>
> Signed-off-by: Álvaro Begué <alvaro.begue@gmail.com>
> ---
>  libstdc++-v3/src/c++20/tzdb.cc                |  9 +--
>  .../std/time/time_zone/numeric_save.cc        | 58 +++++++++++++++++++
>  2 files changed, 63 insertions(+), 4 deletions(-)
>  create mode 100644
> libstdc++-v3/testsuite/std/time/time_zone/numeric_save.cc
>
> diff --git a/libstdc++-v3/src/c++20/tzdb.cc
> b/libstdc++-v3/src/c++20/tzdb.cc
> index b0fbfc46a..1e49bb749 100644
> --- a/libstdc++-v3/src/c++20/tzdb.cc
> +++ b/libstdc++-v3/src/c++20/tzdb.cc
> @@ -478,11 +478,12 @@ namespace std::chrono
>
>        ZoneInfo(sys_info&& info)
>        : m_buf(std::move(info.abbrev)), m_expanded(true),
> m_save(info.save),
> - m_offset(info.offset), m_until(info.end)
> + m_offset(info.offset - seconds(info.save)), m_until(info.end)
>        { }
>
>        ZoneInfo(const pair<sys_info, string_view>& info)
> -      : m_expanded(true), m_save(info.first.save),
> m_offset(info.first.offset),
> +      : m_expanded(true), m_save(info.first.save),
> + m_offset(info.first.offset - seconds(info.first.save)),
>   m_until(info.first.end)
>        {
>   if (info.second.size())
> @@ -494,7 +495,7 @@ namespace std::chrono
>   m_buf += info.first.abbrev;
>        }
>
> -      // STDOFF: Seconds from UTC during standard time.
> +      // STDOFF: Seconds from UTC during standard time (without any save).
>        seconds
>        offset() const noexcept { return m_offset; }
>
> @@ -539,7 +540,7 @@ namespace std::chrono
>    return false;
>
>   info.end = until();
> - info.offset = offset();
> + info.offset = offset() + seconds(m_save);
>   info.save = minutes(m_save);
>   info.abbrev = format();
>   format_abbrev_str(info); // expand %z
> diff --git a/libstdc++-v3/testsuite/std/time/time_zone/numeric_save.cc
> b/libstdc++-v3/testsuite/std/time/time_zone/numeric_save.cc
> new file mode 100644
> index 000000000..1b5b22263
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/std/time/time_zone/numeric_save.cc
> @@ -0,0 +1,58 @@
> +// { 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* } }
> +
> +// When a Zone line specifies a numeric value as its RULES field, that
> +// value is the constant DST save value for that zone line.  Per
> +// [time.zone.info.sys] sys_info::offset is the total UTC offset
> +// (stdoff + save).
> +
> +#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 "./";
> +  }
> +}
> +
> +int
> +main()
> +{
> +  using namespace std::chrono;
> +
> +  std::ofstream("tzdata.zi") << R"(# version test_numeric_save
> +Z Test/Gaborone 2 -  CAT  1943 Sep 19 2
> +                2 1  CAST 1944 Mar 19 2
> +                2 -  CAT
> +)";
> +
> +  const auto& db = reload_tzdb();
> +  VERIFY( override_used );
> +  VERIFY( db.version == "test_numeric_save" );
> +
> +  auto* tz = locate_zone("Test/Gaborone");
> +
> +  // Sample well inside the CAST (numeric-save) zone line.
> +  auto info = tz->get_info(sys_days(1943y/December/15));
> +  VERIFY( info.offset == 3h );        // stdoff +2h + save +1h
> +  VERIFY( info.save == 60min );
> +  VERIFY( info.abbrev == "CAST" );
> +
> +  // Bordering zone lines should report the standard offset with save 0.
> +  auto before = tz->get_info(sys_days(1943y/September/1));
> +  VERIFY( before.offset == 2h );
> +  VERIFY( before.save == 0min );
> +  VERIFY( before.abbrev == "CAT" );
> +
> +  auto after = tz->get_info(sys_days(1944y/April/15));
> +  VERIFY( after.offset == 2h );
> +  VERIFY( after.save == 0min );
> +  VERIFY( after.abbrev == "CAT" );
> +}
> --
> 2.34.1
>
>
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