The port from hand-rolled function storage to std::function
inadvertently removed the helper variable of static storage duration
whose dtor would unregister the conversion function. This caused
QTBUG-94831, where the cleanup of conversion functions attempts to
call code (via std::function) from a library that has already been
unloaded.
Restore the 5.15 behavior by adding a static-storage-duration scope
guard to unregister the conversion and view functions from Qt upon
library unload (when static objects are destroyed). Unlike 5.15, only
install the scope guard upon successful registration, ensuring that
only the DLL which successfully registered its conversion function
unregisters it again.
Amends 0e4ae4fbf8.
Add some strategic std::move()s as a drive-by.
Pick-to: 6.2
Task-number: QTBUG-94831
Change-Id: I391ca667420cf0d98a166676b9bb363d6e190306
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
fa7f4c5efd