[LWG3272] %I %p can parse/format duration since midnight.

[tomaszkam]

Original comment:

%I %p can parse/format duration since midnight.

[HowardHinnant]

This one is a question of how do these flags react with durations as opposed to time points, and is the existing wording already clear on this point.

Example program:

#include "date/date.h"
#include <iostream>
#include <sstream>
#include <string>

int
main()
{
    using namespace date;
    using namespace std;
    using namespace std::chrono;
    // Format
    {
    // Case 1
    // format time_point with %I%p
    cout << format("%F %I%p", sys_days{2019_y/August/10}+14h) << '\n';
    }
    {
    // Case 2
    // format duration with %I%p
    cout << format("%I%p", 14h) << '\n';
    }

    // Parse
    {
    // Case 1
    // Parse %I%p as day-of-year combined with an hour into a time_point
    istringstream in{"2019-08-10 2pm"};
    system_clock::time_point tp;
    in >> parse("%F %I%p", tp);
    cout << tp << '\n';
    }
    {
    // Case 2
    // Parse %I%p as number of hours into a duration
    istringstream in{"2pm"};
    system_clock::duration d;
    in >> parse("%I%p", d);
    cout << d << '\n';
    }
}

Expected output:

2019-08-10 02PM
02PM
2019-08-10 14:00:00.000000
50400000000µs

[tomaszkam]

I struggled with this and the %j issues because from my reading of wording it is unclear which format/parse specifiers are expected to work with durations and which with time points. But I have little experience with POSIX format spec.

[HowardHinnant]

Mailed to LWG chair.