My code has always compiled correctly in Xcode versions prior to 12. Now that I'm trying to build in 12 I'm getting this error - which I know how to fix, except that it's in the CoreController header, GCDevice.h, provided by Apple.
Doubly annoyingly, I'm not even using CoreController - but it's imported by GameKit.h, which I do use.
My code is written in ObjectiveC, and built for MacOS, and GameKit is imported as follows (if that makes any difference):

```
#import <Cocoa/Cocoa.h>
#import <GameKit/GameKit.h>
#import <AVFoundation/AVAudioPlayer.h>

```

When I create a completely new project, with GameKit and with only my code which uses GameKit copied and pasted into it (so not the actual game), it builds fine. Which, to my mind, suggests maybe a problem with the build settings?

GameKit is only imported once, so I don't think that there's a problem with multiple imports. Has anyone else seen this issue?