initializer_list for arrays with predefined sizes

So let's imagine we want to use c++'s initializer_list for some user-defined type that represents array of predefined size (know at compile time). For example matrix's indexes (row,column):

#include <initializer_list>
#include <algorithm>
#include <assert.h>

using idx_t = unsigned;

class MatrixIdx {
public:
    MatrixIdx() :
        _idx { 0, 0 }
    {}

    // this is how I'm able to use initializer list for now
    // cons:
    //     * run-time assertion
    //     * additional header (algorithm)
    //     * not in member initializer list (call me perfectionist)
    //
    MatrixIdx(std::initializer_list<idx_t> l)
    {
        assert(l.size() == 2);
        std::copy(l.begin(), l.end(), _idx);
    }

    //I want to achive constructor similiar to this one
    //Won't compile
    //
    //MatrixIdx(std::initializer_list<idx_t> l) :
        //static_assert(l.size() == 2, "two elements expected") //compile time assertion
        //_idx[0] ( l[0] ),
        //_idx[1] ( l[1] )
    //{}

    idx_t row() { return _idx[0]; }
    void setRow(const idx_t i) { _idx[0] = i; }

    idx_t col() { return _idx[1]; }
    void setCol(const idx_t i) { _idx[1] = i; }

private:
   idx_t _idx[2];
};

int main(void)
{
    MatrixIdx idx; //default - OKAY

    //MatrixIdx idxIL { 1, 2 }; //this should compile

    //this should emit human-readable compile-time error 
    //MatrixIdx idxILError { 1, 2, 3 };

    return 0;
}

What will be the best possible way to achieve this?