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, 2 deletions

diff --git a/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp b/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
index b63867ec3d..6b7dda8afe 100644
--- a/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
+++ b/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
@@ -4036,13 +4036,16 @@ void tst_qqmlecmascript::signalWithJSValueInVariant_twoEngines()
     object->setProperty("compare", compare);
     object->setProperty("pass", false);
 
-    QTest::ignoreMessage(QtWarningMsg, "JSValue can't be reassigned to another engine.");
+    const bool isManaged = QJSValuePrivate::asManagedType<QV4::Managed>(&value) != nullptr;
+
+    if (isManaged)
+        QTest::ignoreMessage(QtWarningMsg, "JSValue can't be reassigned to another engine.");
     emit object->signalWithVariant(QVariant::fromValue(value));
     if (expression == "undefined")
         // if the engine is wrong, we return undefined to the other engine,
         // making this one case pass
         return;
-    QVERIFY(!object->property("pass").toBool());
+    QCOMPARE(object->property("pass").toBool(), !isManaged);
 }
 
 void tst_qqmlecmascript::signalWithQJSValue_data()