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, 2 insertions, 1 deletions

diff --git a/src/quick/items/context2d/qquickcanvasitem.cpp b/src/quick/items/context2d/qquickcanvasitem.cpp
index bfee190baf..a15533418c 100644
--- a/src/quick/items/context2d/qquickcanvasitem.cpp
+++ b/src/quick/items/context2d/qquickcanvasitem.cpp
@@ -59,6 +59,7 @@
 #include <private/qv4scopedvalue_p.h>
 #include <private/qv4jscall_p.h>
 #include <private/qv4qobjectwrapper_p.h>
+#include <private/qjsvalue_p.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -381,7 +382,7 @@ void QQuickCanvasItem::setContextType(const QString &contextType)
 QJSValue QQuickCanvasItem::context() const
 {
     Q_D(const QQuickCanvasItem);
-    return d->context ? QJSValue(d->context->v4Engine(), d->context->v4value()) : QJSValue();
+    return d->context ? QJSValuePrivate::fromReturnedValue(d->context->v4value()) : QJSValue();
 }
 
 /*!