explicit object accessing a protected member

Question

I'm trying to understand why this code will not compile:

class A {
   public:
    template <class Self>
    void invokeUser(this Self&& self) {
        self.invokeProtected();
    }
};

template <typename... Bases>
class B : public Bases... {
   protected:
    void invokeProtected() {}
};

int main() { B<A>{}.invokeUser(); }

It fails with the following error:

error: 'invokeProtected' is a protected member of 'B<A>'
note: in instantiation of function template specialization 'A::invokeUser<B<A>>'

It clearly sees it's being called with a B<A> reference, which is our "this" or "self" in this case. From that context we should be able to get access to the protected members, right? Is this something that the standard prohibits, or is this a clang bug?

Answer 1

Member access checks are restrictions on uses of names to certain scopes.

A is not B, nor is it a subclass of B, nor a friend of B, so all the protected members of B are inaccessible within A. It doesn't matter that self is an explicit object argument.