From "How to write unmaintainable code" [0], here is a function declaration that changes signature based on how many times the header is #included:

    #ifndef DONE
    #ifdef TWICE
    void g(char* str);
    #define DONE
    #else // TWICE
    #ifdef ONCE
    void g(void* str);
    #define TWICE
    #else // ONCE
    void g(std::string str);
    #define ONCE
    #endif // ONCE
    #endif // TWICE
    #endif // DONE

Granted, it isn't one line long.

[0] https://www.thc.org/root/phun/unmaintain.html (Cert issue shows up on FF unfortunately)