Interdependent class template and std::is_base_of specialization

Question

I am mildy confused by the following situation where I have a specialization enabled on is_base_of.

is_base_of requires the full definition of

Answer 1

std::is_base_of requires complete type. which is not the case in

template 
struct derived : base
{
        child child_; // derived not yet complete here.
        typedef T type; 
};

For T::base_tag, IIRC (I think POI of child move from before struct derived to current location in the class), T doesn't need to be complete, and only visited part of it is visible.

So derived::type would not be visible. (but derived::base::type would be).