We can do it!

main.cpp

#define SOL_CHECK_ARGUMENTS
#define SOL_NO_RTTI
#define SOL_NO_EXCEPTIONS // Just to prove that not having these on will still make it work

#include "sol.hpp"

struct A {
	int a = 0xA; virtual int bark() { return 1; }
};

std::tuple<int, int> bark(int num_value, A* a) {
	return std::tuple<int, int>( num_value * 2, a->bark() );
}

int main(int argc, char* const argv[]) {
	sol::state lua;
	
	lua.new_usertype<A>( "A",
		"bark", &A::bark );
	
	lua.script(R"( function f (num_value, a)
	return num_value * 2, a:bark()
end 
nested = { variables = { no = { problem = 10 } } } )");
	lua.set_function("g", bark);

	sol::function cpp_bark = lua["f"];
	sol::function lua_bark = lua["g"];
	sol::reference lua_variable_x = lua["nested"]["variables"]["no"]["problem"];
	A cpp_variable_y;
	
	std::tuple<int, int> cppresult = cpp_bark(lua_variable_x, cpp_variable_y);
	std::pair<int, int> luaresult = lua_bark(lua_variable_x, cpp_variable_y);
	
	return 0;
}