Undefined reference while calling a static constexpr array of variadic types

I have the following class

template<int ... Args>
struct foo {
    constexpr static int arr[sizeof...(Args)]={Args...};
    constexpr int dimension(int i) {return arr[i];}
};

But I get undefined reference to arr, while calling dimension. If I move arr inside the function dimension then the function cannot be a constexpr anymore, because it requires two semicolons within the body of the function. For instance, I cannot do

constexpr int a = foo_obj.dimension(2);

My goal is to metaprogrammatically iterate over all the dimensions of a varidic template and compare it to another integral number? Ideally if I have two objects of foo I want to determine if they are equal in every dimension.