矩阵添加:“operator+”的重载不明确

我有一个矩阵类，我希望能够：

1） 添加2个矩阵，C = A + B;

2） 添加矩阵和标量C = a + B;或C = A + b;

我收到一个警告/错误（取决于编译器标志），说函数调用不明确：

g++ test.cpp -std=c++11
test.cpp: In function ‘int main()’:
test.cpp:111:16: error: ambiguous overload for ‘operator+’ (operand types are ‘matrix<double>’ and ‘matrix<int>’)
     auto C = A + B;
                ^
test.cpp:111:16: note: candidates are:
test.cpp:56:55: note: matrix<decltype ((declval<T>() + declval<U>()))> matrix<T>::operator+(const matrix<U>&) const [with U = int; T = double; decltype ((declval<T>() + declval<U>())) = double]
 matrix<decltype(std::declval<T>()+std::declval<U>())> matrix<T>::operator+(const matrix<U> &B) const
                                                       ^
test.cpp:76:57: note: matrix<decltype ((declval<V>() + declval<U>()))> operator+(const U&, const matrix<V>&) [with V = int; U = matrix<double>; decltype ((declval<V>() + declval<U>())) = matrix<double>]
 matrix<decltype(std::declval<V>() + std::declval<U>())> operator+(const U &a, const matrix<V> &B)
                                                         ^
test.cpp:90:57: note: matrix<decltype ((declval<V>() + declval<U>()))> operator+(const matrix<U>&, const V&) [with V = matrix<int>; U = double; decltype ((declval<V>() + declval<U>())) = matrix<double>]
 matrix<decltype(std::declval<V>() + std::declval<U>())> operator+(const matrix<U> &A, const V &b)

以下是MWE:

#include <iostream>
#include <vector>
#include <array>
template<class T>
class matrix
{
public:
    matrix();
    matrix(const size_t &d1, const size_t &d2);
    T& operator()(const size_t &i, const size_t &j);
    T operator()(const size_t &i, const size_t &j) const;
    template<class U>
    matrix<decltype(std::declval<T>()+std::declval<U>())> operator+(const matrix<U> &B) const;
    size_t size(const size_t &n) const;
private:
    std::vector<T> mData;
    std::array<size_t,2> mSize;
};
template<class T>
matrix<T>::matrix() : mData()
{
    for(size_t ii = 0; ii < 2; ii++)
    {
        mSize[ii] = 0;
    }
}
template<class T>
matrix<T>::matrix(const size_t & d1, const size_t & d2) : mData()
{
    mSize[0] = d1;
    mSize[1] = d2;
    mData.resize(d1*d2);
}
template<class T>
T& matrix<T>::operator()(const size_t &i, const size_t &j)
{
    return mData[j*mSize[0] + i];
}
template<class T>
T matrix<T>::operator()(const size_t &i, const size_t &j) const
{
    return mData[j*mSize[0] + i];
}
template<class T>
template<class U>
matrix<decltype(std::declval<T>()+std::declval<U>())> matrix<T>::operator+(const matrix<U> &B) const
{
    matrix<decltype(std::declval<T>()+std::declval<U>())> C(mSize[0],mSize[1]);
    for(size_t ii = 0; ii < mSize[0]; ii++)
    {
        for(size_t jj = 0; jj < mSize[1]; jj++)
        {
            C(ii,jj) = this->mData[jj*mSize[0] + ii] + B(ii,jj);
        }
    }
    return C;
}
template<class T>
size_t matrix<T>::size(const size_t &n) const
{
    return mSize[n-1];
}
template<class V, class U>
matrix<decltype(std::declval<V>() + std::declval<U>())> operator+(const U &a, const matrix<V> &B)
{
    matrix<decltype(std::declval<V>() + std::declval<U>())> C(B.size(1),B.size(2));
    for(size_t ii = 0; ii < B.size(1); ii++)
    {
        for(size_t jj = 0; jj < B.size(2); jj++)
        {
            C(ii,jj) = a + B(ii,jj);
        }
    }
    return C;
}
template<class V, class U>
matrix<decltype(std::declval<V>() + std::declval<U>())> operator+(const matrix<U> &A, const V &b)
{
    matrix<decltype(std::declval<V>() + std::declval<U>())> C(A.size(1),A.size(2));
    for(size_t ii = 0; ii < A.size(1); ii++)
    {
        for(size_t jj = 0; jj < A.size(2); jj++)
        {
            C(ii,jj) = A(ii,jj) + b;
        }
    }
    return C;
}
int main()
{
    matrix<double> A(3,3);
    A(0,0) = 1.5;
    matrix<int> B(3,3);
    B(0,0) = 1;
    auto C = A + B;
    std::cout << C(0,0) << std::endl;
}

问题：为什么我会收到此警告/错误（取决于编译器标志）？如何在不更改所需界面的情况下修复此问题？