Xcode 7.3.1 autocomplete code completion not working
Without doing anything, In Build-Settings -> Deployment changing iOS Deployment target from 8.0 to 8.1 resolved the issue for me.
It also works for XCode 6 as mentioned here
Also works on Xcode 8, Simply change the deployment target from iOS 9.0 to iOS 9.1.
Xcode 8 has now fixed this very annoying autocomplete issue. So anyone who is still having this problem, update to Xcode 8.
Edit: Please note, this has not solved for others. So your milage may vary. For me, it has fixed without doing anything extra. But they are using their new 'intelligent' autocomplete. Thinking about it, it could be part of the indexing. Deleting the DerivedData folder could help.
Perhaps, this answer is too late, I'll share it anyway.
First of all, I've uninstalled Alcatraz and all plugins.
Second, I've followed ElonChan's instructions explained on:
As a result, autocompletion automagically got back! Thanks @ElonChan.
I hope this answer also solves your problem.