QDate: make conversion from/to chrono types constexpr and noexcept

Add method stdSysDaysToJulianDay(), which implements the conversion
from std::chrono::sys_days to Julian Day in a constexpr and noexcept
manner. Use it in the fromStdSysDays() method instead of addDays(),
and also in all constructors from std::chrono types.
This allows all constructors to be constexpr and noexcept.

Simplify toStdSysDays() so that it could also be constexpr and
noexcept.

Add compile-time roundtrip tests between QDate and std::chrono types.

Fixes: QTBUG-118221
Change-Id: Id35a669430162f1932ad8d28b553af2fd7f5c6de
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 32 insertions, 0 deletions

diff --git a/tests/auto/corelib/time/qdate/tst_qdate.cpp b/tests/auto/corelib/time/qdate/tst_qdate.cpp
index 8da22918a7..2ab928ef82 100644
--- a/tests/auto/corelib/time/qdate/tst_qdate.cpp
+++ b/tests/auto/corelib/time/qdate/tst_qdate.cpp
@@ -1745,6 +1745,38 @@ void tst_QDate::roundtrip() const
         QCOMPARE(loopDate.toJulianDay(), testDate.toJulianDay());
         loopDate = loopDate.addDays(1);
     }
+
+#if __cpp_lib_chrono >= 201907L
+    // Test roundtrip for from/to std::chrono conversions.
+    // Compile-time test, to verify it's all constexpr.
+    using namespace std::chrono;
+    {
+        constexpr sys_days expected{days{minJd}};
+        constexpr sys_days actual{QDate::fromStdSysDays(expected).toStdSysDays()};
+        static_assert(actual == expected);
+    }
+    {
+        // constexpr year_month_day expected{sys_days{days{maxJd}}}; // Overflow at least on MSVC
+        constexpr year_month_day expected{1970y, January, 1d};
+        constexpr sys_days actual{QDate(expected).toStdSysDays()};
+        static_assert(actual == sys_days(expected));
+    }
+    {
+        constexpr year_month_day_last expected{2001y, {October / last}};
+        constexpr sys_days actual{QDate(expected).toStdSysDays()};
+        static_assert(actual == sys_days(expected));
+    }
+    {
+        constexpr year_month_weekday expected{2001y, December, Saturday[1]};
+        constexpr sys_days actual{QDate(expected).toStdSysDays()};
+        static_assert(actual == sys_days(expected));
+    }
+    {
+        constexpr year_month_weekday_last expected{2001y, November, Friday[last]};
+        constexpr sys_days actual{QDate(expected).toStdSysDays()};
+        static_assert(actual == sys_days(expected));
+    }
+#endif // __cpp_lib_chrono >= 201907L
 }
 
 void tst_QDate::qdebug() const