message: Fix specialization of is_error_code_enum
  Explicit specialization of a struct outside its namespace requires
  nested-name-specifier.
  Bug
diff: @@ -106,8 +106,11 @@ inline error_category const& event_category()
  return result;
  }
  +namespace system
  +{
  /// Specialization of \p is_error_code_enum for \p event_errc.
  template<> struct is_error_code_enum<event_errc> : true_type {};
  +} // end system
  /// \return <tt>error_code(static_cast<int>(e), event_category())</tt>
  inline error_code make_error_code(event_errc e)
  