QDateTime: future-proof swap()

In Qt 7 32-bit builds, the ShortData will be larger than the
d-pointer.

So don't swap() the d-pointer, but a) delegate to Data to swap itself
and b) swap the ShortData there, adding a static_assert in the .cpp
file that triggers when the assumption that ShortData is always at
least as large as a pointer is violated.

Found while porting away from overly-generic qSwap(), so done that,
too.

Task-number: QTBUG-97601
Pick-to: 6.3 6.2
Change-Id: I5139da58d99e9491a582ff2cb2f817cd96952044
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qdatetime.cpp

@@ -3617,6 +3617,7 @@ QDateTime::QDateTime() noexcept
     static_assert(sizeof(ShortData) == sizeof(qint64));
     static_assert(sizeof(Data) == sizeof(qint64));
 #endif
+    static_assert(sizeof(ShortData) >= sizeof(void*), "oops, Data::swap() is broken!");
 }
 
 /*!

src/corelib/time/qdatetime.h

@@ -267,6 +267,9 @@ class Q_CORE_EXPORT QDateTime
         Data &operator=(const Data &other);
         ~Data();
 
+        void swap(Data &other) noexcept
+        { std::swap(data, other.data); }
+
         bool isShort() const;
         void detach();
 
@@ -290,7 +293,7 @@ public:
     QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_PURE_SWAP(QDateTime)
     QDateTime &operator=(const QDateTime &other) noexcept;
 
-    void swap(QDateTime &other) noexcept { qSwap(d.d, other.d.d); }
+    void swap(QDateTime &other) noexcept { d.swap(other.d); }
 
     bool isNull() const;
     bool isValid() const;

tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp

@@ -48,6 +48,7 @@ public Q_SLOTS:
 private Q_SLOTS:
     void ctor();
     void operator_eq();
+    void moveSemantics();
     void isNull();
     void isValid();
     void date();
@@ -332,6 +333,18 @@ void tst_QDateTime::operator_eq()
     QVERIFY(dt1 == dt2);
 }
 
+void tst_QDateTime::moveSemantics()
+{
+    QDateTime dt1{QDate{2004, 3, 24}, QTime{23, 45, 57}, Qt::UTC};
+    QDateTime dt2{QDate{2005, 3, 11}, QTime{0, 0}, Qt::UTC};
+    QDateTime copy = dt1;
+    QDateTime moved = std::move(dt1);
+    QCOMPARE(copy, moved);
+    copy = dt2;
+    moved = std::move(dt2);
+    QCOMPARE(copy, moved);
+}
+
 void tst_QDateTime::isNull()
 {
     QDateTime dt1;