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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/114244] Need to use round when parsing fractional
 seconds
Date: Thu, 07 Mar 2024 00:30:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114244

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yup, the seconds part "00.002" is parsed using std::numpunct (in order to
handle the locale's decimal point) and then converted to milliseconds using
duration_cast:

                          auto& __ng =3D use_facet<num_get<_CharT>>(__loc);
                          long double __val;
                          ios_base::iostate __err2{};
                          __ng.get(__buf, {}, __buf, __err2, __val);
                          if (__is_failed(__err2)) [[unlikely]]
                            __err |=3D __err2;
                          else
                            {
                              duration<long double> __fs(__val);
                              __s =3D duration_cast<_Duration>(__fs);
                            }


We also use duration_cast when parsing %c and %X using std::time_get, but
that's an integer conversion there (but the duration_cast should have been
qualified to prevent ADL):

                          __h =3D hours(__tm.tm_hour);
                          __min =3D minutes(__tm.tm_min);
                          __s =3D duration_cast<_Duration>(seconds(__tm.tm_=
sec));

and another duration_cast in chrono::from_stream for durations. That one co=
uld
be used with either integral or floating-point reps.

Do we want to parse "00:00:31" as minutes(1)? I don't think we do, so only =
the
first case where converting long double to milliseconds should be rounded?=