return false;
    }
    if (out_edge.second != src_oidx_) {
      if (listener->IsInterested()) {
        *listener << "\nexpected output slot to be " << src_oidx_
                  << " but found " << out_edge.second;
      }
      return false;
    }

    return true;
  }

  void DescribeTo(::std::ostream* os) const override {
    if (src_oidx_) {
      *os << "output slot: " << src_oidx_ << ", source: (";