I have just changed my app from supporting iOS 8 and up to supporting iOS 9 and up. I believe I've done this properly, since when I now build my app, I'm getting warnings for everything in the app which was deprecated in iOS 9.
I am using Xcode 9, GM seed.
The problem is that I cannot enable the "Use Safe Area Layout Guides" toggle in any of my storyboards. When I do, I get a warning "Safe Area Layout Guide before iOS 9.0". Is there some additional setting that I need to update? A clean and build did not fix the problem.
I didn't try quitting Xcode and restarting it before posting to SO! Sorry! The answer is, quit Xcode and try again.