Conversion from derived* to base* exists but is inaccessible

Why does the following code produce this error even though c is a struct and has public inheritance by default?

struct c 
{
protected:
    int i;
public:
    c(int ii = 0) : i(ii) {}
    virtual c* fun();
};

c* c::fun() 
{
    cout << "in c";
    return &c();
}

class d : c
{
public:
    d() {}
    d* fun()
    {
        i = 9;
        cout << "in d" << '\t' << i;
        return &d();
    }
};

int main()
{
    c *cc;
    d dd;
    cc = &dd;
    cc->fun();
}