Class hash

tensorflow/core/lib/hash/hash.h:134–136

// The second type argument is only used for SFNIAE below.
template <typename T, typename = void>
struct hash {
  size_t operator()(const T& t) const { return std::hash<T>()(t); }
};

template <typename T>
struct hash<T, typename std::enable_if<std::is_enum<T>::value>::type> {