Some of the new iOS 10 features contain link-time checks, that is they are only enabled if the app was linked to the iOS 10 SDK. When working on an open source library it is necessary to debug when linked to an older SDK, e.g. iOS 9.3, on a newer device iOS, e.g. iOS 10. It currently isn’t possible to deploy to a newer iOS device with the old Xcode designed for the older SDK. For this reason it can be necessary to enable the older SDK in the newer Xcode, which can be done as follows:
Note: these commands assume Xcode 7 is at \Applications\Xcode.app and Xcode 8 is at \Applications\Xcode-beta.app
sudo ln -s /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk /Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS9.3.sdk sudo ln -s /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk /Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator9.3.sdk sudo /usr/libexec/PlistBuddy -c "Set :MinimumSDKVersion 9.3" /Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/Info.plist sudo /usr/libexec/PlistBuddy -c "Set :MinimumSDKVersion 9.3" /Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Info.plist
After running these commands, restart Xcode 8. Now you can set the Base SDK to iOS 9.3 and deploy to an iOS 10 device.
Any newer code generation features in Xcode 8 will need to be turned off, e.g. a Core Data model will need to have code generation set to none.