template specialization -->error: explicit specialization of non-template struct 'GameObjectImpl'

I am writing a Game & I have a interface called GameObject.
This game object is implemented by derived objects say planet , comet , asteroids etc. Now I need type_index of these objects in different functions.

To achieve this I define virtual function in GameObject & want to use template pattern with type of derived class . unfortunately I am getting error error: explicit specialization of non-template struct 'GameObjectImpl' template struct GameObjectImpl : GameObject ^

Below is the source code

struct GameObject
{
    virtual ~GameObject() = default;
    virtual type_index type() const = 0;
};
template <typename T> struct GameObjectImpl<T> : GameObject //error
{
    type_index type() const override
    {
        return typeid(T);
    }
};

struct Planet : GameObjectImpl<Planet>{};
struct Asteriod : GameObjectImpl<Asteriod>{};

Can you please let me know how to get rid of this error.Pl note i am learning template pattern so wants to use only template