Ambiguous base class conversion with a compressed pair

So I tried creating a compressed pair using the empty base optimization. I would like it such that if class a and b are empty then compressed_pair<a, b> is empty as well. So I defined my compressed pair something like this:

template <class First, class Second>
struct compressed_pair : First, Second 
{
    compressed_pair() {}
    compressed_pair(const First& x, const Second & y)
    : First(x), Second(y) 
    {}
    First& first()  { return *this; }
    Second& second() { return *this; }
};

However, if one of the types inherit from the other it becomes ambiguous. For example, when I compile this program:

struct a
{};

struct b : a
{};

int main()
{
    compressed_pair<a, b> p;
    auto x = p.first();
}

I get this error from clang:

compressed_pair.cpp:8:30: error: ambiguous conversion from derived class 'compressed_pair<a, b>' to base class 'a':
    struct compressed_pair<struct a, struct b> -> struct a
    struct compressed_pair<struct a, struct b> -> struct b -> struct a
    First& first()  { return *this; }
                             ^~~~~
compressed_pair.cpp:21:16: note: in instantiation of member function 'compressed_pair<a, b>::first' requested here
    auto x = p.first();
               ^

So how can I avoid the ambiguous conversion and still have compressed_pair<a, b> be empty?