Xcode 10 Broke Autocomplete When Inserting Include Names for C++, Any Workarounds?
I can confirm same behaviour on my machine. But you can have the autocompletion for C++ back if you add the include path specified in Build Settings -> Search Path -> Header Search Path. See the picture below.



For all/many C++ projects you may add/modify the Custom Paths in Xcode Preferences-> Locations -> Custom Paths.