I'm adding a class that declares a swap function in its .h file:

  class PayloadUnion {
   public:
    ...
    friend void swap(PayloadUnion& a, PayloadUnion& b);

and defines it in its .cc file:

  void swap(PayloadUnion& a, PayloadUnion& b) {
    using std::swap;
    swap(a.Audio, b.Audio);
    swap(a.Video, b.Video);
    swap(a.is_audio_, b.is_audio_);
  }

Presubmit (specifically, cpplint.py) correctly complained that the .cc file needs to #include <utility> since it uses std::swap.

However, it also complains that the .h file needs to #include <utility>. AFAICT this is wrong, since the .h file makes no reference to std::swap.

(CL where the problem appeared: https://webrtc-review.googlesource.com/c/src/+/4340)