Derived class cannot access protected member during instance creation

Question

I want the class to create an alias usingfrom the return type of one of its class member functions protected. However, the following fails:

#include <type_traits>
class B
{
protected:
    int fun(){return 0;}
};

class D : protected B
{
    using T = decltype(std::declval<B>().fun());
};

main() {}

prog.cpp:5:6: error: ‘int B::fun()’ is protected
  int fun(){return 0;}

Living example

Why Ddoesn’t have access to the protected method of its base?