KaMPIng does not work with types which define a data() member but are no buffers.

[niklas-uhl]

Consider a user (me) wants to send the following type of element:

struct Packet {
  int         rank_;
  std::byte   data_[PACKET_SIZE];
  std::pair<int, int> address_;


  int rank() const {
    return rank_;
  }

  void rank(int rank) {
    rank_ = rank;
  }

  std::pair<int, int> address() const {
    return address_;
  }

  void address(std::pair<int, int> address) {
    address_ = address;
  }

  auto& data() {
    return data_;
  }
  auto const& data() const {
    return data_;
  }
};

With this, passed by const-ref to send_buf, we get a compile error, because KaMPIng assumes that it is a buffer.
We should therefore not only check for the data() member but also for the other properties of C++20's std::ranges::contiguous_range.