The document says "you can specialize the object type with containers like tsl::ordered_map or nlohmann::fifo_map". I had a try with simple code like:

#include "fifo_map.hpp"
#include "json.hpp"

using json = nlohmann::basic_json<nlohmann::fifo_map>;

int main()
{
    json j;
}

It fails the compilation noisily. Would you please fix the issue, and/or clarify in the document how this should be done?