Problem with serialization of my custom template doubly-linked list #1881

Im trying to write a function responsible for saving my custom list into json file.

The list i want to save into file consists of Node objects (Node is inner class of List). The list i need to save uses Doctor objects as T ( List is template class defined in single header file).

Following the guide, I've implemented toJson method for Doctor, and toJson method for my list:
`
template < class T >
void List::toJson(nlohmann::json& j, const List< T >& list) {

j = json::array();
for (Node* current = list.head; current != nullptr; current = current->getNext()) {
	nlohmann::json temp = current->getData();
	j.push_back(temp);
}

}`

(*) getData() method returns reference to Doctor object stored in Node.

Now, in separate class JsonWriter i use method (List.h is included in JsonWriter.h)

void JsonWriter::saveDoctorList(const List< Doctor >& doctorList)
{

file.open("doctorList.json", std::ios::out);
if (file.good()) {
	nlohmann::json myArray = doctorList;
	
}			

}'
...and that's the point where i get stuck. Inside if statement, the compilers signals that
"no suitable user-defined conversion from "const List< Doctor >" to "nlohmann::json" exists".

When i try to do the same nlohmann::json myArray = doctorList; inside my List.h file it compiles without an error.

I've tried to find solution on both github and StackOverflow. I've also looked through issues here, but I can't find anything that solves my problems. Can anybody tell me what am I missing?

Using VS 2017 and Windows

Thanks for any suggestions.