read_as_json Eigen::Matrix support

[elijahfrey]

I am a newer programmer and still learning c++ so apologies in advance in any of my terminology is wrong. I am using glaze in a project to parse input files in json format and came across an issue when trying to overwrite elements of an Eigen::Matrix using the read_as_json function. My usage looks something like:

glz::read_as_json(input_struct, "/matrix/0", "1.0")

where matrix is a member of the input_struct. This ends up returning a seek error and the value is not overwritten. The issue occurs because when the json_ptr is cut down to "/0" the default implementation of op() in the seek_op struct is called and since json_ptr is not empty an error is returned. I was able to fix the issue by adding:

template <class T>
   requires(eigen_t<T> && !array_t<T>)
struct seek_op<T> 
{
  template <class F>
  static bool op(F&& func, auto&& value, sv json_ptr)
  {
    if (json_ptr.empty()) {
      func(value)
      return true;
    {
    if (json_ptr[0] != '/' || json_ptr.size() < 2) return false;

    size_t index{};
    auto [p, ec] = std::from_chars(&json_ptr[1], json_ptr.data() + json_ptr.size(), index);
    if (ec != std::errc{}) return false;
    json_ptr = json_ptr.substr(p - json_ptr.data());
    
    if (index >= value.size()) return false;
    return seek(std::forward<F>(func), value(index), json_ptr);
  }
};

into seek.hpp. Not sure if this is a bug, feature that has not yet been implemented, or something intentionally left out so I thought I would bring it up in discussions. Additionally, this issue only came up when I used a 2d eigen matrix, when I used eigen vectors no issues occured.