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

[Jonathan Wakely]

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().

But it's really just because int{unsigned{__ym}} /would/ give a
narrowing warning, right?

Would either int(unsigned(__my)) or static_cast<int>(__ym) make sense
instead, do avoid people wondering about it in future?


> 	  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
>