[PATCH] libstdc++: Fix arithmetic bug in year_month_weekday conversion [PR96713]

[Jonathan Wakely]

On 28/10/20 11:04 -0400, Patrick Palka via Libstdc++ wrote:
>The conversion function year_month_weekday::operator sys_days computes
>the number of days to offset from the first weekday of the month with:
>
> days{(index()-1)*7}
>      ^~~~~~~~~~~~~  type 'unsigned'
>
>We'd like 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 long instead yields a large positive quantity.
>
>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 that we do the right thing when index() == 5.
>
>Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

Yes, thanks.


>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.
>---
> libstdc++-v3/include/std/chrono               |  3 +-
> .../std/time/year_month_weekday/3.cc          | 66 +++++++++++++++++++
> 2 files changed, 68 insertions(+), 1 deletion(-)
> create mode 100644 libstdc++-v3/testsuite/std/time/year_month_weekday/3.cc
>
>diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
>index 7539d7184ea..7c35b78fe59 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{((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 00000000000..86db85d04e2
>--- /dev/null
>+++ b/libstdc++-v3/testsuite/std/time/year_month_weekday/3.cc
>@@ -0,0 +1,66 @@
>+// { 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;
>+  using ymwd = year_month_weekday;
>+
>+  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);
>+}
>-- 
>2.29.0.rc0
>