V4 Engine: Don't try to convert JS functions to other types

When converting a JS value to a variant, if we notice that we get a
QJSValue again, there is no point in trying to convert it further. We'll
just run into infinite recursion.

Pick-to: 6.3
Fixes: QTBUG-102545
Change-Id: I0a40e21287e5460e5e214101aabe8d2b4bf0afad
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/qml/jsruntime/qv4engine.cpp

@@ -168,7 +168,10 @@ ReturnedValue throwTypeError(const FunctionObject *b, const QV4::Value *, const
 template <typename ReturnType>
 ReturnType convertJSValueToVariantType(const QJSValue &value)
 {
-    return value.toVariant().value<ReturnType>();
+    const QVariant variant = value.toVariant();
+    return variant.metaType() == QMetaType::fromType<QJSValue>()
+            ? ReturnType()
+            : variant.value<ReturnType>();
 }
 
 struct JSArrayIterator {

tests/auto/qml/qjsvalue/tst_qjsvalue.cpp

@@ -2871,6 +2871,10 @@ void tst_QJSValue::jsFunctionInVariant()
         QTest::ignoreMessage(QtDebugMsg, "direct call");
         log.callWithInstance(console, {"direct call"});
     }
+
+    const QVariant var = log.toVariant();
+    QCOMPARE(var.metaType(), QMetaType::fromType<QJSValue>());
+    QCOMPARE(var.value<QVariantMap>(), QVariantMap()); // Does not recurse infinitely
 }
 
 void tst_QJSValue::integerToEnum()