Seems odd to have an unused method. Moreover, the function is fragile and dangerous, because one could have a std::vector vec_a and type CDataStream{vec_a, 0, 0}.size() and CDataStream{0, 0, vec_a}.size(), assuming they are the same thing, when in fact they are not. (The first takes over the memory as is, the second serializes the vector).
So my suggestion would be to remove the unused method and introduce a new method when this functionality is needed. For example: static DataStream FromMany(Args&&... args).