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>

src/corelib/time/qdatetime.cpp

@@ -446,14 +446,14 @@ QDate::QDate(int y, int m, int d, QCalendar cal)
 }
 
 /*!
-    \fn QDate::QDate(std::chrono::year_month_day ymd)
+    \fn QDate::QDate(std::chrono::year_month_day date)
-    \fn QDate::QDate(std::chrono::year_month_day_last ymd)
+    \fn QDate::QDate(std::chrono::year_month_day_last date)
-    \fn QDate::QDate(std::chrono::year_month_weekday ymd)
+    \fn QDate::QDate(std::chrono::year_month_weekday date)
-    \fn QDate::QDate(std::chrono::year_month_weekday_last ymd)
+    \fn QDate::QDate(std::chrono::year_month_weekday_last date)
 
     \since 6.4
 
-    Constructs a QDate representing the same date as \a ymd. This allows for
+    Constructs a QDate representing the same date as \a date. This allows for
     easy interoperability between the Standard Library calendaring classes and
     Qt datetime classes.
 
@@ -462,9 +462,9 @@ QDate::QDate(int y, int m, int d, QCalendar cal)
     \snippet code/src_corelib_time_qdatetime.cpp 22
 
     \note Unlike QDate, std::chrono::year and the related classes feature the
-    year zero. This means that if \a ymd is in the year zero or before, the
+    year zero. This means that if \a date is in the year zero or before, the
     resulting QDate object will have an year one less than the one specified by
-    \a ymd.
+    \a date.
 
     \note This function requires C++20.
 */

src/corelib/time/qdatetime.h

@@ -32,53 +32,36 @@ public:
     QDate(int y, int m, int d, QCalendar cal);
 #if __cpp_lib_chrono >= 201907L || defined(Q_QDOC)
     QT_POST_CXX17_API_IN_EXPORTED_CLASS
-    Q_IMPLICIT QDate(std::chrono::year_month_day ymd)
+    Q_IMPLICIT constexpr QDate(std::chrono::year_month_day date) noexcept
+        : jd(date.ok() ? stdSysDaysToJulianDay(date) : nullJd())
+    {}
+
+    QT_POST_CXX17_API_IN_EXPORTED_CLASS
+    Q_IMPLICIT constexpr QDate(std::chrono::year_month_day_last date) noexcept
+        : jd(date.ok() ? stdSysDaysToJulianDay(date) : nullJd())
+    {}
+
+    QT_POST_CXX17_API_IN_EXPORTED_CLASS
+    Q_IMPLICIT constexpr QDate(std::chrono::year_month_weekday date) noexcept
+        : jd(date.ok() ? stdSysDaysToJulianDay(date) : nullJd())
+    {}
+
+    QT_POST_CXX17_API_IN_EXPORTED_CLASS
+    Q_IMPLICIT constexpr QDate(std::chrono::year_month_weekday_last date) noexcept
+        : jd(date.ok() ? stdSysDaysToJulianDay(date) : nullJd())
+    {}
+
+    QT_POST_CXX17_API_IN_EXPORTED_CLASS
+    static constexpr QDate fromStdSysDays(const std::chrono::sys_days &days) noexcept
     {
-        if (!ymd.ok())
+        return QDate(stdSysDaysToJulianDay(days));
-            jd = nullJd();
-        else
-            *this = fromStdSysDays(ymd);
     }
 
     QT_POST_CXX17_API_IN_EXPORTED_CLASS
-    Q_IMPLICIT QDate(std::chrono::year_month_day_last ymdl)
+    constexpr std::chrono::sys_days toStdSysDays() const noexcept
     {
-        if (!ymdl.ok())
+        const qint64 days = isValid() ? jd - unixEpochJd() : 0;
-            jd = nullJd();
+        return std::chrono::sys_days(std::chrono::days(days));
-        else
-            *this = fromStdSysDays(ymdl);
-    }
-
-    QT_POST_CXX17_API_IN_EXPORTED_CLASS
-    Q_IMPLICIT QDate(std::chrono::year_month_weekday ymw)
-    {
-        if (!ymw.ok())
-            jd = nullJd();
-        else
-            *this = fromStdSysDays(ymw);
-    }
-
-    QT_POST_CXX17_API_IN_EXPORTED_CLASS
-    Q_IMPLICIT QDate(std::chrono::year_month_weekday_last ymwl)
-    {
-        if (!ymwl.ok())
-            jd = nullJd();
-        else
-            *this = fromStdSysDays(ymwl);
-    }
-
-    QT_POST_CXX17_API_IN_EXPORTED_CLASS
-    static QDate fromStdSysDays(const std::chrono::sys_days &days)
-    {
-        const QDate epoch(unixEpochJd());
-        return epoch.addDays(days.time_since_epoch().count());
-    }
-
-    QT_POST_CXX17_API_IN_EXPORTED_CLASS
-    std::chrono::sys_days toStdSysDays() const
-    {
-        const QDate epoch(unixEpochJd());
-        return std::chrono::sys_days(std::chrono::days(epoch.daysTo(*this)));
     }
 #endif
 
@@ -167,6 +150,21 @@ private:
     static constexpr inline qint64 maxJd() { return Q_INT64_C( 784354017364); }
     static constexpr inline qint64 unixEpochJd() { return Q_INT64_C(2440588); }
 
+#if __cpp_lib_chrono >= 201907L
+    static constexpr qint64 stdSysDaysToJulianDay(const std::chrono::sys_days &days) noexcept
+    {
+        const auto epochDays = days.time_since_epoch().count();
+        // minJd() and maxJd() fit into 40 bits.
+        if constexpr (sizeof(epochDays) >= 41) {
+            constexpr auto top = maxJd() - unixEpochJd();
+            constexpr auto bottom = minJd() - unixEpochJd();
+            if (epochDays > top || epochDays < bottom)
+                return nullJd();
+        }
+        return unixEpochJd() + epochDays;
+    }
+#endif // __cpp_lib_chrono >= 201907L
+
     qint64 jd;
 
     friend class QDateTime;

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