std::enable_if With Non-Type Template Parameters

It all depends on what kind of error/failure you want to raise on invalid code. Here it is one possibility (leaving aside the obvious static_assert(Width==Height, "not square matrix");)

(C++98 style)

#include<type_traits>
template<int Width, int Height, typename T>
class Matrix{
public:
    template<int WDummy = Width, int HDummy = Height>
    static typename std::enable_if<WDummy == HDummy, Matrix>::type
    Identity(){
        Matrix ret;
        for (int y = 0; y < Width; y++){
        // elements[y][y] = T(1);
        }
        return ret;
    }
};

int main(){
    Matrix<5,5,double> m55;
    Matrix<4,5,double> m45; // ok
    Matrix<5,5, double> id55 = Matrix<5,5, double>::Identity(); // ok
//  Matrix<4,5, double> id45 = Matrix<4,5, double>::Identity(); // compilation error! 
//     and nice error: "no matching function for call to ‘Matrix<4, 5, double>::Identity()"
}

---

EDIT: In C++11 the code can be more compact and clear, (it works in clang 3.2 but not in gcc 4.7.1, so I am not sure how standard it is):

(C++11 style)

template<int Width, int Height, typename T>
class Matrix{
public:
    template<typename = typename std::enable_if<Width == Height>::type>
    static Matrix
    Identity(){
        Matrix ret;
        for(int y = 0; y < Width; y++){
            // ret.elements[y][y] = T(1);
        }
        return ret;
    }
};

---

EDIT 2020: (C++14)

template<int Width, int Height, typename T>
class Matrix{
public:
    template<typename = std::enable_if_t<Width == Height>>
    static Matrix
    Identity()
    {
        Matrix ret;
        for(int y = 0; y < Width; y++){
        //  ret.elements[y][y] = T(1);
        }
        return ret;
    }
};

---

(C++20) https://godbolt.org/z/cs1MWj

template<int Width, int Height, typename T>
class Matrix{
public:
    static Matrix
    Identity()
        requires(Width == Height)
    {
        Matrix ret;
        for(int y = 0; y < Width; y++){
        //  ret.elements[y][y] = T(1);
        }
        return ret;
    }
};