Cannot put a QObject* into a shared pointer after putting it into a qobject-tracking qweakpointer or a QPointer

This segfaults:

#include <QtCore>

int main(int argc, char **argv)
{
  QObject *o = new QObject;
  QWeakPointer<QObject> wp( o );
  QSharedPointer<QObject> sp( o );
}

QPointer<T> does not segfault.

Because QPointer in Qt 5 is implemented in terms of QWeakPointer, this problem manifests itself in several more places. Some of the consequences of this can be seen using git grep QPointer. A non-exhaustive list:

• Putting the result of myPluginLoader::instance() into a QSharedPointer aborts (this is true since Qt 4.7.0)

• Can't put a model into a QSharedPointer after putting it into a QItemSelectionModel

• Can't put any QObject into a QSharedPointer after using it as an event filter with installEventFilter.

From there the effect snowballs.

Because of holding a QPointer:

• Can't put a QNetworkAccessManager into a QSharedPointer after creating a QNetworkReply.

• Can't create an asyncronous DBus call with callback and then put the receiver into a QSharedPointer.

Because of the event filter issue:

• Can't put a running QStateMachine which contains a QEventTransition into a QSharedPointer.

• Can't put a QFontDialog into a QSharedPointer (also not much reason to, but...)

These are just the obvious effects of this change in Qt 5.

There will certainly be more subtle side-effects and breakages resulting from implementing QPointer using QWeakPointer.

The attached tests pass in Qt 4.8 and all fail with Qt 5.