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Avoid arithmetic overflow in the constructors of weekday #5156

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Merged
Stephan T. Lavavej (StephanTLavavej) merged 2 commits into from
Dec 5, 2024
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Avoid arithmetic overflow in the constructors of weekday #5156

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Avoid arithmetic overflow in the constructors of `weekday`
frederick-vs-ja Nov 29, 2024
f535837
Casey's code review suggestions
CaseyCarter Nov 30, 2024
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6 changes: 4 additions & 2 deletions stl/inc/chrono
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Expand Up @@ -473,10 +473,12 @@ namespace chrono {
private:
unsigned char _Weekday;

// courtesy of Howard Hinnant
// courtesy of Howard Hinnant (modified to avoid overflow)
// https://howardhinnant.github.io/date_algorithms.html#weekday_from_days
_NODISCARD static constexpr unsigned int _Weekday_from_days(int _Tp) noexcept {
return static_cast<unsigned int>(_Tp >= -4 ? (_Tp + 4) % 7 : (_Tp + 5) % 7 + 6);
_STL_INTERNAL_STATIC_ASSERT(~0u % 7u == 3u); // offset for `_Tp < 0` needs to change
const auto _Before_modulo = static_cast<unsigned int>(_Tp) + (_Tp >= 0 ? 4u : 0u);
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return _Before_modulo % 7u; // separate expression for MSVC codegen, see GH-5153
}
};

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4 changes: 4 additions & 0 deletions tests/std/tests/P0355R7_calendars_and_time_zones_dates/test.cpp
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Expand Up @@ -281,6 +281,10 @@ constexpr void weekday_test() {
assert(Sunday - Monday == days{6});
assert(Sunday - Tuesday == days{5});
assert(Wednesday - Thursday == days{6});

// GH-5153 "<chrono>: integer overflow in weekday::weekday(sys_days::max())"
assert(weekday{sys_days::max()} == weekday{sys_days::max() - days{7}});
assert(weekday{local_days::max()} == weekday{local_days::max() - days{7}});
}

constexpr void weekday_indexed_test() {
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