[std::unexpected](http://en.cppreference.com/w/cpp/error/unexpected) was removed from C++17. RXCPP already [removed the call](https://github.com/Reactive-Extensions/RxCpp/commit/fea65be680234ba0677b84223de5563716b5c1aa), but most recent [release](https://github.com/Reactive-Extensions/RxCpp/releases) is older than that.