[PATCH] libstdc++: Fix arithmetic bug in chrono::year_month::operator+

[Jonathan Wakely]

On 27/08/20 12:37 -0400, Patrick Palka wrote:
>On Thu, 27 Aug 2020, Jonathan Wakely wrote:
>
>> On 27/08/20 11:29 -0400, Patrick Palka via Libstdc++ wrote:
>> > This fixes the months-based addition for year_month when the
>> > year_month's month component is zero.
>> >
>> > Successfully tested on x86_64-pc-linux-gnu, on the 'date' library's
>> > calendar and (now) on libcxx's calendar tests.  Does this look OK to
>> > commit?
>> >
>> > libstdc++-v3/ChangeLog:
>> >
>> > 	* include/std/chrono (year_month::operator+): Properly handle a
>> > 	month value of 0 by casting the month value to int before
>> > 	subtracting 1 from it so that the difference is signed in the
>> > 	subsequent addition.
>> > 	* testsuite/std/time/year_month/1.cc: Test addition of months to
>> > 	a year_month whose month value is below and above the normalized
>> > 	range of [1,12].
>> > ---
>> > libstdc++-v3/include/std/chrono                 |  2 +-
>> > libstdc++-v3/testsuite/std/time/year_month/1.cc | 12 ++++++++++++
>> > 2 files changed, 13 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/libstdc++-v3/include/std/chrono
>> > b/libstdc++-v3/include/std/chrono
>> > index 417954d103b..398008c8f31 100644
>> > --- a/libstdc++-v3/include/std/chrono
>> > +++ b/libstdc++-v3/include/std/chrono
>> > @@ -2133,7 +2133,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> > 	{
>> > 	  // TODO: Optimize?
>> > 	  auto __m = __ym.month() + __dm;
>> > -	  auto __i = unsigned{__ym.month()} - 1 + __dm.count();
>> > +	  auto __i = int(unsigned{__ym.month()}) - 1 + __dm.count();
>>
>> The mix of paren-init and braced-init makes me do a double-take.
>>
>> It makes me stop and wonder if there is some reason? Maybe to check
>> for narrowing? But it can't narrow since it just calls
>> year_month::operator unsigned().
>
>[time.cal] seems to mostly use braced-init for converting a calendar
>type to the underlying type, and I mindlessly followed suit :) e.g. in
>[time.cal.year.nonmembers] and [time.cal.day.nonmembers].
>
>Ah, but there's [time.cal.month.nonmembers]/7 which uses a static_cast
>instead.
>
>>
>> But it's really just because int{unsigned{__ym}} /would/ give a
>> narrowing warning, right?
>
>Hmm yes, looks like it.
>
>>
>> Would either int(unsigned(__my)) or static_cast<int>(__ym) make sense
>> instead, do avoid people wondering about it in future?
>
>The first form works for me.  The second form is rejected it seems.
>
>Does the following look OK to commit?

OK, thanks.


>-- >8 --
>
>Subject: [PATCH] libstdc++: Fix arithmetic bug in
> chrono::year_month::operator+
>
>This fixes the months-based addition for year_month when the
>year_month's month component is zero.
>
>libstdc++-v3/ChangeLog:
>
>	* include/std/chrono (year_month::operator+): Properly handle a
>	month value of 0 by casting the month value to int before
>	subtracting 1 from it so that the difference is sign-extended in
>	the subsequent addition.
>	* testsuite/std/time/year_month/1.cc: Test adding months to a
>	year_month whose month component is below or above the
>	normalized range of [1,12].
>---
> libstdc++-v3/include/std/chrono                 |  2 +-
> libstdc++-v3/testsuite/std/time/year_month/1.cc | 12 ++++++++++++
> 2 files changed, 13 insertions(+), 1 deletion(-)
>
>diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
>index 417954d103b..f0fa66c7a2b 100644
>--- a/libstdc++-v3/include/std/chrono
>+++ b/libstdc++-v3/include/std/chrono
>@@ -2133,7 +2133,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> 	{
> 	  // TODO: Optimize?
> 	  auto __m = __ym.month() + __dm;
>-	  auto __i = unsigned{__ym.month()} - 1 + __dm.count();
>+	  auto __i = int(unsigned(__ym.month())) - 1 + __dm.count();
> 	  auto __y = (__i < 0
> 		      ? __ym.year() + years{(__i - 11) / 12}
> 		      : __ym.year() + years{__i / 12});
>diff --git a/libstdc++-v3/testsuite/std/time/year_month/1.cc b/libstdc++-v3/testsuite/std/time/year_month/1.cc
>index 007cfeb2f72..4c331dcdb50 100644
>--- a/libstdc++-v3/testsuite/std/time/year_month/1.cc
>+++ b/libstdc++-v3/testsuite/std/time/year_month/1.cc
>@@ -83,4 +83,16 @@ constexpr_year_month()
>   static_assert(2017y/33 + months{0} == 2019y/9);
>
>   static_assert(2010y/January + months{-12} == 2009y/January);
>+
>+  static_assert(2010y/month{0} + months{-1} == 2009y/November);
>+  static_assert(2010y/month{0} + months{0} == 2009y/December);
>+  static_assert(2010y/month{0} + months{1} == 2010y/January);
>+  static_assert(2010y/month{0} + months{2} == 2010y/February);
>+  static_assert(2010y/month{0} + months{11} == 2010y/November);
>+  static_assert(2010y/month{0} + months{12} == 2010y/December);
>+  static_assert(2010y/month{0} + months{13} == 2011y/January);
>+
>+  static_assert(months{-1} + 2010y/month{37} == 2012y/December);
>+  static_assert(months{0} + 2010y/month{37} == 2013y/January);
>+  static_assert(months{1} + 2010y/month{37} == 2013y/February);
> }
>-- 
>2.28.0.337.ge9b77c84a0

>