Xcode 12.5 troubleshooting guide (RN 0.61/0.62/0.63/0.64)

Problem

Hey folks. This issue wants to be a follow up to https://github.com/facebook/react-native/issues/31179; that issue has become really long so opening a new one to try and reduce the noise and confusion around it. Here is how to fix the Xcode 12.5 iOS build breaks with errors related to "atomic_wait_until is unavailable" (and similar messages). (sidenote: make sure you are using cocoapods 1.10.x) If you are on 0.61 Note well: this has only been tested by a few members of the community, not by me, so I can't ensure it will work. If you are still using RN 0.61, you can refer to this comment https://github.com/facebook/react-native/issues/28405#issuecomment-840914650 from @edgarbonillag. If you are on 0.62 1. add this line to your podfile (or modify it if you already had it): [code block] 2. change your package.json react-native dependency to [code block] 3. do all the classic necessary cleans (node_modules, caches, pod folders, etc)(react-native-clean-project is your ally) then do `yarn install` and a `pod install --repo-update` (if pod install fails on an error about a Flipper package, just remove the relevant lines from the podfile.lock and run the pod install again) If you are on 0.63 Note well: you need to be on latest 0.63.x release, at the time of writing 0.63.3. If you are on 0.63, you are in luck! You just need to add this line to your podfile (or modify it if you already had it): [code block] Then properly clean your caches (react-native-clean-project is you