Use constructor parameters outside constructor in constexpr class

I am trying to implement matrix-like class, using an std::array to actually store the data. All of the data is known at compile-time.

I want to be able to use initializer-lists to initialize the Matrix. Something along the lines of

Matrix m = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

I also want to be able to set the dimensions of my matrix during instantiation.

My current code is similar to the following:

template<int m, int n>
class Matrix {
public:

    using initializer_list2d = std::initializer_list< std::initializer_list<float> >;
    using array2d = std::array< std::array<float, m>, n >;

    constexpr Matrix(initializer_list2d initList) {
        // static_assert to check initList length [...]

        int i = 0;
        for (auto &row : initList) {
            // static_assert to check row length [...]

            std::copy(row.begin(), row.end(), data_[i].begin());
            i++;
        }
    }

    // Definitions of operators and methods [...]
private:
    array2d data_;
};

In order to use this though, I have to set the dimensions through the template:

Matrix<3, 3> m = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

It seems to me, that since the constructor is constexpr (and all of its parameters have to be known at compile-time anyway) it should somehow bet possible to deduce the dimensions through the initializer-list alone.

Using a dynamic data type (e.g. vector) is not possible for my application, since this program has to be able to run without access to the heap.

Is there some way to achieve this?

Thanks for any help in advance!