If the "Addressable" option to ValueTypeBehavior is set, you can use the "as" operator to cast a previously unknown type into either undefined or the given type. We can use this in qmlcachegen to generate efficient code for further operations on the same type. In the generated C++ it in fact only works for GetLookup because: a, We generally don't do SetLookup on value types, yet. b, We generally don't call methods on value types, yet. c, We cannot store a union of undefined and a sequence type, yet. However, getting properties of value types is the most important application of the new casts so this is well worth it. As a side effect we can also look up things in potentially undefined results of other operations now. For example list lookups. Task-number: QTBUG-94807 Change-Id: Ifdf34f1f3f67b7a0a8953b9ed0e947b74638a28c Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>