QVariant::toDouble and similar methods don't work with custom types

template <typename T>
inline T qNumVariantToHelper(const QVariant::Private &d,
                             const HandlersManager &handlerManager, bool *ok, const T& val)
{
	const uint t = qMetaTypeId<T>();
	if (ok)
		*ok = true;
	if (d.type == t)
		return val;
	T ret = 0;
	if ((d.type >= QMetaType::User || t >= QMetaType::User)
	        && QMetaType::convert(&val, d.type, &ret, t)) {
		return ret;
	}
	if (!handlerManager[d.type]->convert(&d, t, &ret, ok) && ok)
		*ok = false;
	return ret;
}

#include <QCoreApplication>
#include <QMetaType>
#include <QVariant>
#include <iostream>
struct MyCustomType {
	int a;
	double b;
};
Q_DECLARE_METATYPE(MyCustomType)
int main(int argc, char *argv[])
{
	QCoreApplication app(argc, argv);
	QMetaType::registerConverter<MyCustomType, double>([](const MyCustomType &val){return val.b;});
	QVariant a = QVariant::fromValue(MyCustomType{1000,3.14});
	double d = a.value<double>(); // works fine
	std::cout << d << std::endl;
	bool ok = true;
	double dd = a.toDouble(&ok); // doesn't work
	std::cout << "ok is " << (ok ? "true" : "false") << std::endl;
	std::cout << dd << std::endl;
	return 0;
}