libstdc++: Fix numeric save offset on Zone lines [PR124851]

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(0 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.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Signed-off-by: Álvaro Begué <alvaro.begue@gmail.com>

diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index 587e88bc0dd51095a661d2f17a8c4b1b46f19f92..886d7e71d8eb9515e6ac456f2baf454dcb5dd618 100644 (file)
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -482,12 +482,13 @@ 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_until(info.first.end)
+      : 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())
          {
@@ -498,7 +499,7 @@ 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; }
 
@@ -543,7 +544,7 @@ 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 (file)
index 0000000..687524b
--- /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 ); // If this fails then XFAIL for the target.
+  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" );
+}