On Tue, 6 Oct 2026 at 11:06, Jonathan Wakely <[email protected]> wrote:
>
> On Tue, 6 Oct 2026 at 11:01, Jonathan Wakely <[email protected]> wrote:
> >
> > Formatting utc_time can produce a time ending with ":60" for a leap
> > second, so we should also be able to read that back using chrono::parse.
> >
> > We should reject ":60" unless the time really is a leap second, so
> > compare to the list of known leap seconds. Continue to reject ":60" for
> > any time point except utc_time.
> >
> > libstdc++-v3/ChangeLog:
> >
> > PR libstdc++/126543
> > * include/bits/chrono_io.h (from_stream(..., utc_time&, ...)):
> > Use chrono::get_leap_second_info to check for valid leap
> > seconds.
> > (_Parser::operator()) <R>: Do not accept "60" for minutes.
> > <S>: Accept "60" for seconds.
> > * testsuite/std/time/parse/114279.cc: Use a real leap second as
> > the test input. Check parsing whole seconds as well as
> > fractional seconds.
> > * testsuite/std/time/parse/126543.cc: New test.
> > ---
> >
> > Tested x86_64-linux.
> >
> > v2: Split out the changes to only set *abbrev and *offset if the entire
> > parse succeeds in to PATCH v2 2/2. So 1/2 only implements the fixes to
> > properly parse and validate leap seconds.
> >
> > libstdc++-v3/include/bits/chrono_io.h | 12 ++++--
> > .../testsuite/std/time/parse/114279.cc | 28 +++++++++----
> > .../testsuite/std/time/parse/126543.cc | 40 +++++++++++++++++++
> > 3 files changed, 70 insertions(+), 10 deletions(-)
> > create mode 100644 libstdc++-v3/testsuite/std/time/parse/126543.cc
> >
> > diff --git a/libstdc++-v3/include/bits/chrono_io.h
> > b/libstdc++-v3/include/bits/chrono_io.h
> > index eec12357fdf7..5cb9c159e5b5 100644
> > --- a/libstdc++-v3/include/bits/chrono_io.h
> > +++ b/libstdc++-v3/include/bits/chrono_io.h
> > @@ -3961,7 +3961,11 @@ namespace __detail
> > // "23:59:60" to correctly produce a time within a leap second.
> > auto __ut = utc_clock::from_sys(__p._M_sys_days) + __p._M_time
> > - *__offset;
> > - __tp = __detail::__round<_Duration>(__ut);
> > + if (__p._M_is_leap_second
> > + && !chrono::get_leap_second_info(__ut).is_leap_second)
>
> We could optimize this slightly by checking if:
>
> (__p._M_time - *__offset) == 23h + 59min + 60s
>
> and only doing the get_leap_second_info call if that's true. Because
> it must be true for valid leap seconds, so we avoid the lookup (and
> potentially slow call into the library to read a new leapseconds file)
> on input like 01:23:60 which can never be a valid leap second.
i.e. like this
if (__p(__is, __fmt, __abbrev, __offset))
{
// Converting to utc_time before adding _M_time is necessary for
// "23:59:60" to correctly produce a time within a leap second.
auto __ut = utc_clock::from_sys(__p._M_sys_days) + __p._M_time
- *__offset;
if (__p._M_is_leap_second
&& (chrono::floor<seconds>(__p._M_time - *__offset) != hours(24)
|| !chrono::get_leap_second_info(__ut).is_leap_second))
__is.setstate(ios_base::failbit);
else
__tp = __detail::__round<_Duration>(__ut);
> Or we could do that inside _Parser::operator() where we set
> _M_is_leap_second and set failbit.
