It's the declaration of a static method. And I think the problem is Player. Cuz I'm getting another error "Player: unidentified identifier" in the same file on this:
std::vector<Player> PlayerList;
It's a member of a different class from method FilterInput().
The struct Player is declared and defined in another header file. And that files has been #include d via a header file that includes some general header files including the file containing struct Player.