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

[Patrick Palka <ppalka@redhat.com>]

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();
 	  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