[Bug libstdc++/126543] New: chrono::parse does not reject "60" seconds for a non-leap second

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126543

            Bug ID: 126543
           Summary: chrono::parse does not reject "60" seconds for a
                    non-leap second
           Product: gcc
           Version: 16.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

#include <chrono>
#include <sstream>
#include <assert.h>

int main()
{
  std::istringstream in;
  std::chrono::utc_seconds ut;
  std::chrono::sys_seconds st;

  in.str("2016-12-31 23:59:60");
  in >> parse("%F %T", ut);
  assert(in.good()); // is a leap second

  in.str("2016-12-31 23:59:60");
  in >> parse("%F %T", st);
  assert(in.fail()); // can't read leap seconds into sys_time
  in.clear();

  in.str("2016-12-31 23:58:60");
  in >> parse("%F %T", ut);
  assert(in.fail()); // not a leap second
  in.clear();
}

The last assertion fails, because we allow ":60" for any utc_time, but we
should verify that the parsed time really was a leap second.

"2016-12-31 23:59:60" is valid
"2016-12-31 23:58:60" is not (a leap second can only occur in the last second
of December or the last second of June).