I was trying to convert syntax of old Swift project to new one as per few suggestion on Stack Overflow I tried converting it through Edit > Convert > To latest Synta
Ok, my turn now. In my case I had an existing project. I copied in a few files from another project. One of the files I brought in was SettingsViewController.swift and there was already a file by the same name I was not aware of. The files were in different directories of the project so no duplicate message was shown. The compile/link messages offered no clue to this. I figured out the issue by adding one file at a time to the project (there were 12 total) and the building each time until I found the offending file. I renamed the new file and project builds now.