Implicitly constructing a value from a ReturnedValue muddies the responsibility for ensuring that the gc can find the object. With this change, we disable the implicit conversion. The expectation for lifetime management is now: - If a ReturnedValue is stored on the C++ stack, it must be put into a QV4::Scoped class (or there should be a comment why not doing so is safe). Passing a ReturnedValue to a function should no longer be possible, unless the function takes a ReturnedValue, in which case the expectation is that it stores the value in a place where it can be seen by the gc, before doing anything that could trigger a gc run. Using Value::fromReturnedValue can still be used to pass a Value on, but in that case, the expectation is that there is a comment which explains why this is safe. - If a QV4::Value is obtained from a function call, it ought to be stored in a ScopedValue, too. We currently can't enforce this easily, so this should be checked during code review. A possible way forward would be to disallow returning Values, but that would be a larger change, and is deferred to the future. - If a functions has a QV4::Value parameter, it's the callers' responsibilty to ensure that the gc can find it. Pick-to: 6.9 6.8 6.5 Fixes: QTBUG-131961 Change-Id: Iea055589d35a5f1ac36fe376d4389eb81de87961 Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>