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, 2 deletions

diff --git a/src/qml/qml/qqmltypewrapper.cpp b/src/qml/qml/qqmltypewrapper.cpp
index 7bcc5e9900..c787d4092e 100644
--- a/src/qml/qml/qqmltypewrapper.cpp
+++ b/src/qml/qml/qqmltypewrapper.cpp
@@ -233,7 +233,7 @@ ReturnedValue QQmlTypeWrapper::virtualGet(const Managed *m, PropertyKey id, cons
                 QJSValue scriptSingleton = e->singletonInstance<QJSValue>(type);
                 if (!scriptSingleton.isUndefined()) {
                     // NOTE: if used in a binding, changes will not trigger re-evaluation since non-NOTIFYable.
-                    QV4::ScopedObject o(scope, QJSValuePrivate::convertedToValue(v4, scriptSingleton));
+                    QV4::ScopedObject o(scope, QJSValuePrivate::asReturnedValue(&scriptSingleton));
                     if (!!o)
                         return o->get(name);
                 }
@@ -349,7 +349,7 @@ bool QQmlTypeWrapper::virtualPut(Managed *m, PropertyKey id, const Value &value,
         } else {
             QJSValue scriptSingleton = e->singletonInstance<QJSValue>(type);
             if (!scriptSingleton.isUndefined()) {
-                QV4::ScopedObject apiprivate(scope, QJSValuePrivate::convertedToValue(scope.engine, scriptSingleton));
+                QV4::ScopedObject apiprivate(scope, QJSValuePrivate::asReturnedValue(&scriptSingleton));
                 if (!apiprivate) {
                     QString error = QLatin1String("Cannot assign to read-only property \"") + name->toQString() + QLatin1Char('\"');
                     scope.engine->throwError(error);