[gcc r13-4100] libstdc++: Add test for chrono::utc_clock leap second offset

[Jonathan Wakely]

https://gcc.gnu.org/g:7026d0455dce1092975d4884f450a12a6ed205c7

commit r13-4100-g7026d0455dce1092975d4884f450a12a6ed205c7
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Nov 16 15:04:32 2022 +0000

    libstdc++: Add test for chrono::utc_clock leap second offset
    
    This test of leap second handling is taken from the C++20 standard.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/std/time/clock/utc/1.cc: Check handling across leap
            second insertion.

Diff:
---
 libstdc++-v3/testsuite/std/time/clock/utc/1.cc | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/libstdc++-v3/testsuite/std/time/clock/utc/1.cc b/libstdc++-v3/testsuite/std/time/clock/utc/1.cc
index eef5f3c3a48..18578fb1ab1 100644
--- a/libstdc++-v3/testsuite/std/time/clock/utc/1.cc
+++ b/libstdc++-v3/testsuite/std/time/clock/utc/1.cc
@@ -9,6 +9,8 @@ test01()
 {
   using namespace std::chrono;
 
+  // [time.clock.utc.overview]
+
   auto epoch = sys_seconds{sys_days{1970y/January/1}};
   auto utc_epoch = clock_cast<utc_clock>(epoch);
   VERIFY( utc_epoch.time_since_epoch() == 0s );
@@ -18,7 +20,29 @@ test01()
   VERIFY( utc_y2k.time_since_epoch() == 946'684'822s );
 }
 
+void
+test02()
+{
+  using namespace std::chrono;
+
+  // [time.clock.utc.members]
+
+  auto t = sys_days{July/1/2015} - 2ns;
+  auto u = utc_clock::from_sys(t);
+  VERIFY(u.time_since_epoch() - t.time_since_epoch() == 25s);
+  t += 1ns;
+  u = utc_clock::from_sys(t);
+  VERIFY(u.time_since_epoch() - t.time_since_epoch() == 25s);
+  t += 1ns;
+  u = utc_clock::from_sys(t);
+  VERIFY(u.time_since_epoch() - t.time_since_epoch() == 26s);
+  t += 1ns;
+  u = utc_clock::from_sys(t);
+  VERIFY(u.time_since_epoch() - t.time_since_epoch() == 26s);
+}
+
 int main()
 {
   test01();
+  test02();
 }