Can i use auto or decltype instead trailing return type?

I find trailing return type so easy to define the return of a function that returns a complicated types e.g:

auto get_diag(int(&ar)[3][3])->int(&)[3]{ // using trailing return type
    static int diag[3]{
        ar[0][0], ar[1][1], ar[2][2]
    };
    return diag;
}

auto& get_diag2(int(&ar)[3][3]){ // adding & auto because otherwise it converts the array to pointer
    static int diag[3]{
        ar[0][0], ar[1][1], ar[2][2]
    };
    return diag;
}

int main(){

    int a[][3]{
        {1, 2, 3},
        {4, 5, 6},
        {7, 8, 9}
    };

    decltype(get_diag(a)) diag{
        get_diag(a)
    };

    for (auto i : diag)
        std::cout << i << ", ";
    std::cout << std::endl;

    decltype(get_diag2(a)) diag2{
        get_diag2(a)
    };

    for (auto i : diag2)
        std::cout << i << ", ";
    std::cout << std::endl;


    std::cout << std::endl;
}

• I want to know what is the difference between the functions get_diag and get_diag2. So as long as the output is the same why I need to use trailing return type?