QCborValue: fix the move-assignment operator

The double-swap technique I used was flawed and broke on
self-assignment. What I had meant to use was the move-and-swap
technique. Thanks to Peppe for pointing it out.

This also fixes a compiler bug in the Green Hills compiler. It was
finding the wrong "swap" function in qSwap:

    using std::swap;
    swap(value1, value2);

It's supposed to find swap(QCborValue &, QCborValue &) due to argument-
dependent lookup. It's instead finding std::swap<QCborValue>, which
recurses.

Fixes: QTBUG-83390
Change-Id: Ibdc95e9af7bd456a94ecfffd1603e1bee90cd107
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/serialization/qcborvalue.h

@@ -188,9 +188,8 @@ public:
     QCborValue &operator=(const QCborValue &other);
     QCborValue &operator=(QCborValue &&other) noexcept
     {
-        QCborValue tmp;
-        qSwap(*this, tmp);
-        qSwap(other, *this);
+        QCborValue tmp(std::move(other));
+        swap(tmp);
         return *this;
     }
 

tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp

@@ -385,9 +385,21 @@ void tst_QCborValue::copyCompare()
 {
     QFETCH(QCborValue, v);
     QCborValue other = v;
+
+    // self-moving
+    v = std::move(v);
+    QCOMPARE(v, other); // make sure it's still valid
+
+    // moving
+    v = std::move(other);
+    other = std::move(v);
+
+    // normal copying
+    other = v;
     other = v;
     v = other;
 
+
     QCOMPARE(v.compare(other), 0);
     QCOMPARE(v, other);
     QVERIFY(!(v != other));