QObjectBindableProperty does not evaluate, even if it has a signal

#include <iostream>

#include <QVariant>
#include <QObject>
#include <QProperty>

class MyClass : public QObject{
    Q_OBJECT
public:
    Q_OBJECT_BINDABLE_PROPERTY(MyClass, QVariant, foo);
};

class EagerClass : public QObject{
Q_OBJECT
signals:
    void mysignal();
public:
    Q_OBJECT_BINDABLE_PROPERTY(EagerClass, QVariant, foo, &EagerClass::mysignal);
};


int main(){
    MyClass a;
    a.foo = 5;

    EagerClass follower;
    follower.foo.setBinding([&](){ return a.foo.value(); });
    QObject::connect(&follower, &EagerClass::mysignal, [](){ std::cout << "Hallo" << std::endl; });

    // uncomment this, then it works
    // follower.foo.value();

    std::cout << "should say Hallo:" << std::endl;
    a.foo = 6;
}

#include "main.moc"