Is it thread safe to use new in meyer's singleton implementation?

We know that Meyers Singleton is thread safe since c++11.

I'm using this singleton pattern.

struct singleton_t
{

  static
  singleton_t &
  instance()
  {
    static singleton_t s;
    return s;
  } // instance

  singleton_t(const singleton_t &) = delete;
  singleton_t & operator = (const singleton_t &) = delete;

private:

  singleton_t() {}
  ~singleton_t() {}

}; // struct singleton_t

singleton_t is not a pointer or reference so the polymorphism doesn't apply.

Now I want to do it something like this:

struct singleton_t
{

  static
  singleton_t *
  instance(singleton_t* p = nullptr)
  {
    static singleton_t *s = nullptr == p ? new singleton_t : p;
    return s;
  } // instance

  singleton_t(const singleton_t &) = delete;
  singleton_t & operator = (const singleton_t &) = delete;

private:

  singleton_t() {}
  ~singleton_t() {}

}; // struct singleton_t

In this way, I singleton_t can point to instance of its subclass so the polymorphism can be achieved. Or is there a better approach?