QObject::setProperty normally checks that the passed-in type can be converted to the target type and doesn't call the setter function if it cannot. This works with user-defined variant types as well: if you pass a wrong type, the setter function won't be called at all.

However, when the custom type is registered with qRegisterMetatype, the check no longer works. If you pass an object of a wrong type to setProperty, the setter function will be called with an object that is filled with zeros. This is fatal for example with classes that contain a d pointer.

The attached project illustrates this. Commenting out qRegisterMetaType() solves the problem.