What is the wrong with my C++ interfacing

[Ferhat Kurtulmuş via Digitalmars-d-learn]

The original C++ class
https://github.com/opencv/opencv/blob/master/modules/core/include/opencv2/core/types.hpp#L315:
template<typename _Tp> class Size_
{
public:
    typedef _Tp value_type;

    //! default constructor
    Size_();
    Size_(_Tp _width, _Tp _height);
    Size_(const Size_& sz);
    Size_(Size_&& sz) CV_NOEXCEPT;
    Size_(const Point_<_Tp>& pt);

    Size_& operator = (const Size_& sz);
    Size_& operator = (Size_&& sz) CV_NOEXCEPT;
    //! the area (width*height)
    _Tp area() const;
    //! aspect ratio (width/height)
    double aspectRatio() const;
    //! true if empty
    bool empty() const;

    //! conversion of another data type.
    template<typename _Tp2> operator Size_<_Tp2>() const;

    _Tp width; //!< the width
    _Tp height; //!< the height
};

// my auxiliary cpp code:
cv::Size_<int>* createSizeIntWH(int _width, int _height){
    return new cv::Size_<int>(_width, _height);
}

void deleteSizeInt(cv::Size_<int> *&sz){
    delete sz;
}

// d code:
extern(C++, cv){
class Size_(_Tp){
    @disable this();

    final _Tp area() const;
    final double aspectRatio() const;
    final bool empty() const;

    _Tp width; //!< the width
    _Tp height; //!< the height
}
}

// my test code that fails:
Size_!int sz = createSizeIntWH(200, 100);
writeln(sz.width);

One of the problems is that sz.width is not printed as 200, but a 
random int.
Other problem is that if I try to call one of area, aspectRatio, 
and empty, it does not compile yielding a linker error:
error LNK2019: unresolved external symbol "public: int __cdecl 
cv::Size_<int>::area(void)const " (?area@?$Size_@H@cv@@QEBAHXZ) 
referenced in function _Dmain
Somehow linker cannot locate that symbol. Is this a name mangling 
issue?
In the same library I could successfully interface cv::Mat which 
is a template-free definition. I suspect if D allows interfacing 
C++ class templates since docs do not cover this, but only struct 
templates?