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libstdc++: Fix arithmetic bug in chrono::year_month::operator+

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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].
This commit is contained in:
Patrick Palka 2020-08-27 14:11:24 -04:00
parent 7b743c67f0
commit 71e9716137
2 changed files with 13 additions and 1 deletions
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2
libstdc++-v3/include/std/chrono
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@ -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});

12
libstdc++-v3/testsuite/std/time/year_month/1.cc
View File

@ -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);
}