[Bug libstdc++/97613] New: chrono::year_month_weekday cast to sys_days : return bad value if index() == 0

[Bug libstdc++/97613] New: chrono::year_month_weekday cast to sys_days : return bad value if index() == 0

[faithandbrave at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97613

            Bug ID: 97613
           Summary: chrono::year_month_weekday cast to sys_days : return
                    bad value if index() == 0
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: faithandbrave at gmail dot com
  Target Milestone: ---

#include <iostream>
#include <chrono>

namespace chrono = std::chrono;
using namespace std::chrono_literals;

int main()
{
  // 1
  chrono::sys_days date1 = 2020y/3/chrono::Sunday[1];
  assert(chrono::year_month_day{date1} == 2020y/3/1);

  // 2
  chrono::sys_days date2 = 2020y/3/chrono::Sunday[2];
  assert(chrono::year_month_day{date2} == 2020y/3/8);

  // 3
  chrono::sys_days date3 = 2020y/3/chrono::Sunday[0];
  assert(chrono::year_month_day{date3} == 2020y/2/23);
}

Assertion failed the program's No.3 patten.
GCC 11 (trunk) returns `30297y/March/14d`. Clang (libc++) is OK.

See the specification:

> 27.8.16.2 [time.cal.ymwd.members]
> If index() is 0 the returned sys_days represents the date 7 days prior to the first weekday() of year()/month().

[Bug libstdc++/97613] chrono::year_month_weekday cast to sys_days : return bad value if index() == 0

[faithandbrave at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97613

--- Comment #1 from Akira Takahashi <faithandbrave at gmail dot com> ---
- #include <iostream>
+ #include <cassert>

#include <cassert>
#include <chrono>

namespace chrono = std::chrono;
using namespace std::chrono_literals;

int main()
{
  chrono::sys_days date1 = 2020y/3/chrono::Sunday[1];
  assert(chrono::year_month_day{date1} == 2020y/3/1);

  chrono::sys_days date2 = 2020y/3/chrono::Sunday[2];
  assert(chrono::year_month_day{date2} == 2020y/3/8);

  chrono::sys_days date3 = 2020y/3/chrono::Sunday[0];
  assert(chrono::year_month_day{date3} == 2020y/2/23);
}

[Bug libstdc++/97613] chrono::year_month_weekday cast to sys_days : return bad value if index() == 0

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97613

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
   Last reconfirmed|                            |2020-10-28

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed.

[Bug libstdc++/97613] chrono::year_month_weekday cast to sys_days : return bad value if index() == 0

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97613

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:8572edc828f6d1e7c8243f901fe7c96f62a11a8e

commit r11-4490-g8572edc828f6d1e7c8243f901fe7c96f62a11a8e
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Oct 28 12:28:08 2020 -0400

    libstdc++: Fix arithmetic bug in year_month_weekday conversion [PR96713]

    The conversion function year_month_weekday::operator sys_days computes
    the offset in days from the first weekday of the month with:

     days{(index()-1)*7}
          ^~~~~~~~~~~~~  type 'unsigned'

    We want the above to yield -7d when index() is 0u, but our 'days' alias
    is based on long instead of int, so the conversion from unsigned to the
    underlying type of 'days' instead yields a large positive value.

    This patch fixes this by casting the result of index() to int so that
    the initializer is sign-extended in the conversion to long.

    The added testcase also verifies we do the right thing when index() == 5.

    libstdc++-v3/ChangeLog:

            PR libstdc++/96713
            * include/std/chrono (year_month_weekday::operator sys_days):
            Cast the result of index() to int so that the initializer for
            days{} is sign-extended when it's converted to the underlying
            type.
            * testsuite/std/time/year_month_weekday/3.cc: New test.

[Bug libstdc++/97613] chrono::year_month_weekday cast to sys_days : return bad value if index() == 0

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97613

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |11.0

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
This should be fixed now, thanks for the report!