[Bug libstdc++/114244] New: Need to use round when parsing fractional seconds

[howard.hinnant at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114244

            Bug ID: 114244
           Summary: Need to use round when parsing fractional seconds
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: howard.hinnant at gmail dot com
  Target Milestone: ---

I'm not sure if this is a bug or just QOI.  But consider:

#include <chrono>
#include <string>
#include <iostream>


using time_point_t = std::chrono::sys_time<std::chrono::milliseconds>;

time_point_t decrypt(std::string s) {
    time_point_t tp;
    std::istringstream in(s);
    in >> parse("%Y%m%d-%T", tp);
    return tp;
}

int main() {
    std::cout << decrypt("20240304-13:00:00.002");
}

I expect the output to be:

2024-03-04 13:00:00.002

but it is:

2024-03-04 13:00:00.001

I believe the issue is the use of duration_cast<milliseconds> or
time_point_cast<milliseconds> somewhere under the parse (not sure exactly
where).  Since 0.002 is not exactly representable in floating point, we're
getting a truncate towards zero because this parses as just barely under 0.002.

I highly recommend the use of round<Duration> when converting floating point
based durations and time_points to integral based.