QmlCompiler: Mark StoreElement on JS values has having side effects

A StoreElement on a JS value can do pretty much anything you can
imagine, after all.

Pick-to: 6.5 6.4 6.4.2
Fixes: QTBUG-109196
Change-Id: Ic638d94b55e6340eb9fe56abc663a6f0f2277f5e
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/qmlcompiler/qqmljstypepropagator.cpp

@@ -681,6 +681,10 @@ void QQmlJSTypePropagator::generate_StoreElement(int base, int index)
         addReadAccumulator(jsValue);
         addReadRegister(base, jsValue);
         addReadRegister(index, jsValue);
+
+        // Writing to a JS array can have side effects all over the place since it's
+        // passed by reference.
+        m_state.setHasSideEffects(true);
         return;
     }
 

tests/auto/qml/qmlcppcodegen/data/CMakeLists.txt

@@ -161,6 +161,7 @@ set(qml_files
     signalIndexMismatch.qml
     signatureIgnored.qml
     specificParent.qml
+    storeElementSideEffects.qml
     stringArg.qml
     stringLength.qml
     stringToByteArray.qml

tests/auto/qml/qmlcppcodegen/data/storeElementSideEffects.qml

@@ -0,0 +1,6 @@
+import QtQml
+
+QtObject {
+    property var myItem: []
+    Component.onCompleted: myItem[0] = 10
+}

tests/auto/qml/qmlcppcodegen/tst_qmlcppcodegen.cpp

@@ -155,6 +155,7 @@ private slots:
     void ambiguousSignals();
     void fileImportsContainCxxTypes();
     void lengthAccessArraySequenceCompat();
+    void storeElementSideEffects();
 };
 
 void tst_QmlCppCodegen::initTestCase()
@@ -2917,6 +2918,21 @@ void tst_QmlCppCodegen::enumConversion()
     QVERIFY(o);
     QCOMPARE(o->property("test").toInt(), 0x04);
     QCOMPARE(o->property("test_1").toBool(), true);
+}
+
+void tst_QmlCppCodegen::storeElementSideEffects()
+{
+    QQmlEngine engine;
+
+    QQmlComponent c(&engine, QUrl(u"qrc:/qt/qml/TestTypes/storeElementSideEffects.qml"_s));
+    QVERIFY2(c.isReady(), qPrintable(c.errorString()));
+
+    QScopedPointer<QObject> o(c.create());
+    QVERIFY(o);
+
+    const QJSValue prop = o->property("myItem").value<QJSValue>();
+    QVERIFY(prop.isArray());
+    QCOMPARE(prop.property(0).toInt(), 10);
 };
 
 void tst_QmlCppCodegen::ambiguousSignals()