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.

2 files changed, 67 insertions, 1 deletions

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index 7539d71..020982d 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -2719,7 +2719,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       operator sys_days() const noexcept
       {
 	auto __d = sys_days{year() / month() / 1};
-	return __d + (weekday() - chrono::weekday(__d) + days{(index()-1)*7});
+	return __d + (weekday() - chrono::weekday(__d)
+		      + days{(static_cast<int>(index())-1)*7});
       }
 
       explicit constexpr
diff --git a/libstdc++-v3/testsuite/std/time/year_month_weekday/3.cc b/libstdc++-v3/testsuite/std/time/year_month_weekday/3.cc
new file mode 100644
index 0000000..cccacce
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/time/year_month_weekday/3.cc
@@ -0,0 +1,65 @@
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// PR libstdc++/97613
+// Test year_month_weekday to sys_days conversion for extreme values of index().
+
+#include <chrono>
+
+void
+test01()
+{
+  using namespace std::chrono;
+  using ymd = year_month_day;
+
+  static_assert(ymd{sys_days{2020y/January/Sunday[0]}} == 2019y/December/29);
+  static_assert(ymd{sys_days{2020y/January/Monday[0]}} == 2019y/December/30);
+  static_assert(ymd{sys_days{2020y/January/Tuesday[0]}} == 2019y/December/31);
+  static_assert(ymd{sys_days{2020y/January/Wednesday[0]}} == 2019y/December/25);
+  static_assert(ymd{sys_days{2020y/January/Thursday[0]}} == 2019y/December/26);
+  static_assert(ymd{sys_days{2020y/January/Friday[0]}} == 2019y/December/27);
+  static_assert(ymd{sys_days{2020y/January/Saturday[0]}} == 2019y/December/28);
+
+  static_assert((2020y).is_leap());
+  static_assert(ymd{sys_days{2020y/March/Sunday[0]}} == 2020y/February/23);
+  static_assert(ymd{sys_days{2020y/March/Monday[0]}} == 2020y/February/24);
+  static_assert(ymd{sys_days{2020y/March/Tuesday[0]}} == 2020y/February/25);
+  static_assert(ymd{sys_days{2020y/March/Wednesday[0]}} == 2020y/February/26);
+  static_assert(ymd{sys_days{2020y/March/Thursday[0]}} == 2020y/February/27);
+  static_assert(ymd{sys_days{2020y/March/Friday[0]}} == 2020y/February/28);
+  static_assert(ymd{sys_days{2020y/March/Saturday[0]}} == 2020y/February/29);
+
+  static_assert(!(2019y).is_leap());
+  static_assert(ymd{sys_days{2019y/March/Sunday[0]}} == 2019y/February/24);
+  static_assert(ymd{sys_days{2019y/March/Monday[0]}} == 2019y/February/25);
+  static_assert(ymd{sys_days{2019y/March/Tuesday[0]}} == 2019y/February/26);
+  static_assert(ymd{sys_days{2019y/March/Wednesday[0]}} == 2019y/February/27);
+  static_assert(ymd{sys_days{2019y/March/Thursday[0]}} == 2019y/February/28);
+  static_assert(ymd{sys_days{2019y/March/Friday[0]}} == 2019y/February/22);
+  static_assert(ymd{sys_days{2019y/March/Saturday[0]}} == 2019y/February/23);
+
+  static_assert(ymd{sys_days{2020y/December/Sunday[5]}} == 2021y/January/3);
+  static_assert(ymd{sys_days{2020y/December/Monday[5]}} == 2021y/January/4);
+  static_assert(ymd{sys_days{2020y/December/Tuesday[5]}} == 2020y/December/29);
+  static_assert(ymd{sys_days{2020y/December/Wednesday[5]}} == 2020y/December/30);
+  static_assert(ymd{sys_days{2020y/December/Thursday[5]}} == 2020y/December/31);
+  static_assert(ymd{sys_days{2020y/December/Friday[5]}} == 2021y/January/1);
+  static_assert(ymd{sys_days{2020y/December/Saturday[5]}} == 2021y/January/2);
+}