message: Add missing override
diff: @@ -86,6 +86,11 @@ public:
  discard();
  }
  + bool disposed() const noexcept override {
  + std::unique_lock guard{mtx_};
  + return mgr_ == nullptr;
  + }
  +
  void on_complete() {
  std::unique_lock guard{mtx_};
  if (sink_) {
  @@ -206,7 +211,7 @@ private:
  }
  }
  - std::recursive_mutex mtx_;
  + mutable std::recursive_mutex mtx_;
  /// Allocated to max_in_flight_ * 2, but at most holds max_in_flight_ elements
  /// at any point in time. We dynamically shift elements into the first half of
  