QtQml: Avoid potential gc issues

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 <[email protected]>

1 files changed, 0 insertions, 3 deletions

diff --git a/src/qml/jsruntime/qv4value_p.h b/src/qml/jsruntime/qv4value_p.h
index 9bbbf63bd0..0f9bbd3b06 100644
--- a/src/qml/jsruntime/qv4value_p.h
+++ b/src/qml/jsruntime/qv4value_p.h
@@ -38,9 +38,6 @@ struct Q_QML_EXPORT Value : public StaticValue
 {
     using ManagedPtr = Managed *;
 
-    Value() = default;
-    constexpr Value(quint64 val) : StaticValue(val) {}
-
     static constexpr Value fromStaticValue(StaticValue staticValue)
     {
         return {staticValue._val};