Feature: Convenience macros for DECLARING to_json and from_json?

[serg06]

I see there is the NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE macro for defining to_json and from_json, but there's nothing for just declaring them, is there?

For example if I want to do something like this:

CustomType.h:

#include "nlohmann/json-fwd.hpp"

struct CustomType
{
	int var1;
	int var2;
};

// DECLARE json conversions
void to_json(nlohmann::json& j, const CustomType& obj);
void from_json(const nlohmann::json& j, CustomType& obj);

CustomType.cpp:

#include "nlohmann/json.hpp"

// DEFINE json conversions
void to_json(nlohmann::json& j, const CustomType& obj)
{
	j["var1"] = obj.var1;
	j["var2"] = obj.var2;
}

void from_json(const nlohmann::json& j, CustomType& obj)
{
	j.at("var1").get_to(obj.var1);
	j.at("var2").get_to(obj.var2);
}

I can use the macro to replace the code in CustomType.cpp, but I still need the to_json and from_json declarations in CustomType.h.