[Development] Behaviour change in QML in Qt 5.8 regarding null

[Allan Sandfeld Jensen]

On Wednesday 28 September 2016, Olivier Goffart wrote:
> 
> int main() {
>     QVariant v1 =  QVariant::fromValue<void*>(nullptr);
>     QVariant v2 =  QVariant::fromValue(nullptr);
>     qDebug() << v1.isNull() << v2.isNull() << (v1 == v2) << (v2 == v1);
>     qDebug() << v2.canConvert<void*>()  << v2.canConvert<QObject*>();
> }
> // output:
> // "false false false false"
> // "false false"
> 
> IMHO this is wrong!
> 
> First of all, I'm not even convinced it makes sens to have std::nullptr_t
> as a builtin type.
> 
> I think any pointer type with nullptr in it should be isNull()
> All we need a specialisation of QVariantIsNull::CallFilteredIsNull for T*
> and for std::nullptr_t.
> That would be a change of behaviour for T* tough. But not for nullptr_t
> since it's new.
> 
> Also I think nullptr_t should be convertible to any pointer type.
> (nullptr_t is converting top any pointer type in the c++ world)
> And that should solve the equality problem.

I would agree, but after looking into the code, I think we have a major 
problem. QVariant::isNull is used for two things, one it maps to isNull on any 
complex classes, but during conversion anything null can't be converted and 
null afterwards indicate a failed conversion. Convert should probably be using 
QVariant::Invalid, but it currently doesn't.

In general we have not separated invalid and null well in the past, and trying 
to fix that will cause issues.

`Allan