[gcc/devel/c++-coroutines] libstdc++: Fix arithmetic bug in chrono::year_month::operator+

[Iain D Sandoe]

https://gcc.gnu.org/g:71e9716137d47872f30f933ff99ba9ef3df1665c

commit 71e9716137d47872f30f933ff99ba9ef3df1665c
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Aug 27 14:11:24 2020 -0400

    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 0.
    
    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].

Diff:
---
 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 df2f5d23f52..9fc8f560d99 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);
 }