Why are access specifiers treated differently when expanding template parameters?

Expanding on the question "Why is decltype'ing members of the parent class forbidden if it's a template?".

Both Clang and GCC complain that B can't access A::member, because it is protected.

But, B can access A::member if a particular instance of B is asked, it's only during the expansion of B<int>::type_name that the error arises.

Why is the public inheritance of A by B ignored in this context? (If that's what's happening.)

template<typename T>
class A{
protected:
   int member;
public:
   using type_name = T;
};

template<typename T>
class B: public A<T>{
   decltype(A<T>::member) another_member;
};

template<typename T,
         typename P=typename T::type_name>
void foo(){}

// Force the instantiation of foo
void bar(){
    foo<B<int>>();
}