Downcasting to grandchildren class, with private inheritance

Question

I am trying to downcast an upcasted pointer, in which the inheritance hierarchy looks lie the following:

A : public Actor
Actor : private ActorImpl

When I save the upcasted pointer of an instance of class A as a pointer to ActorImpl and then when I try to downcast the pointer later back into A, i fail as the pointer after the dynamic cast is a nullptr. I have commented to place where the assertion fails. But the values of std::is_base_of gives me idea that they should not fail.

A* downcast<A>(ActorImpl *ai)
{
    assert(ai != nullptr);

    A *downcasted = nullptr;

    if (std::is_base_of<Actor, A>::value)
    {
        bool grandparent = std::is_base_of<ActorImpl, A>::value;
        assert(grandparent);

        bool parent = std::is_base_of<ActorImpl, Actor>::value;
        assert(parent);

        //this assertion fails even though the A has Actor as parent and ActorImpl as grandparent     
        Actor *downcastedtmp = dynamic_cast<Actor *>(ai);
        assert(downcastedtmp);
        
        downcasted = dynamic_cast<A *>(downcastedtmp);
        assert(downcasted);
    }
    else
    {
        bool bb = std::is_base_of<ActorImpl, A>::value;
        assert(bb);
        downcasted = dynamic_cast<A *>(ai);
        assert(downcasted);
    }

    return downcasted;

What is the reason that this assertion fails? Than you.