Store a QV4::ReturnedValue in QJSValue

Being careful, we can now save primitive values inline. We use the heap
pointer of QV4::Value as either QString* or QV4::Value* for complex
types. We cannot store persistent managed QV4::Value without the double
indirection as those need to be allocated in a special place.

The generic QVariant case is not supported anymore. The only place where
it was actually needed were the stream operators for QJSValue. Those
were fundamentally broken:

* A managed QJSValue saved and loaded from a stream was converted to a
  QVariant-type QJSValue
* QVariant-type QJSValues were not callable, could not be objects or
  arrays, or any of the special types.
* Cyclic references were forcibly broken when saving to a data stream.

In general the support for saving and loading of managed types to/from
a data stream was so abysmally bad that we don't lose much by dropping
it.

[ChangeLog][QML][Important Behavior Changes] When saving a QJSValue to a
QDataStream only primitive values or strings will be retained. Support
for objects and arrays was incomplete and unreliable already before. It
cannot work correctly as we don't necessarily have a JavaScript heap
when loading a QJSValue from a stream. Therefore, we don't have a proper
place to keep any managed values. Using QVariant to keep them instead is
a bad idea because QVariant cannot represent everything a QJSValue can
contain.

Fixes: QTBUG-75174
Change-Id: I75697670639bca8d4b1668763d7020c4cf871bda
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 5 insertions, 3 deletions

diff --git a/src/qml/qml/qqmlbinding.cpp b/src/qml/qml/qqmlbinding.cpp
index b9566d5862..59f2759c81 100644
--- a/src/qml/qml/qqmlbinding.cpp
+++ b/src/qml/qml/qqmlbinding.cpp
@@ -57,6 +57,7 @@
 #include <private/qv4qobjectwrapper_p.h>
 #include <private/qv4variantobject_p.h>
 #include <private/qv4jscall_p.h>
+#include <private/qjsvalue_p.h>
 
 #include <qtqml_tracepoints_p.h>
 
@@ -455,9 +456,10 @@ Q_NEVER_INLINE bool QQmlBinding::slowWrite(const QQmlPropertyData &core,
             delayedError()->setErrorDescription(QLatin1String("Invalid use of Qt.binding() in a binding declaration."));
             return false;
         }
-        QQmlPropertyPrivate::writeValueProperty(m_target.data(), core, valueTypeData, QVariant::fromValue(
-                               QJSValue(v4engine, result.asReturnedValue())),
-                           context(), flags);
+        QQmlPropertyPrivate::writeValueProperty(
+                    m_target.data(), core, valueTypeData,
+                    QVariant::fromValue(QJSValuePrivate::fromReturnedValue(result.asReturnedValue())),
+                    context(), flags);
     } else if (isUndefined) {
         const QLatin1String typeName(QMetaType::typeName(type)
                                      ? QMetaType::typeName(type)