Glad you enjoyed it. If you're pretty certain of this error, you can post a bug/issue at: https://github.com/Codewars/codewars.com/issues
Try to provide any information that may be relevant.

Thanks for the kind words. The Image constructors in C++ are:

Image::Image()
: pixels(NULL), width(0), height(0)
{}

Image::Image(initializer_list<unsigned> data, unsigned w, unsigned h)
: width(w), height(h)
{ 
  pixels = new unsigned[w*h];
  unsigned *p=pixels;
  for(auto d : data)
    *p++ = d;
}

Have you already realised what's wrong in your question? Those params are refer to lambda "sum" function. Not to accumulate.

Thank you!

It seems to be a problem with your class template static member declaration and definition (StackOverflow). Keywords for googling: "c++ class template undefined reference to static member".

Short answer: static data members must be declared and defined (until C++17, which introduces "inline constexpr static members", see reference).