Interesting... I assumed Flow would have issues doing this (and I think it used to), but I was able to run this command without introducing any Flow errors:

cp native/components/clearable-text-input.react.ios.js native/components/clearable-text-input.react.android.js

That confused me, since I was wondering why the React Native repo has two separate .flowconfigs – one for iOS and one for Android.

I did some more investigation and figured out why. When Flow detects variants, it will typecheck each file individually, but when handling imports of that file it will "default" to one of the variants.

So for instance, if you introduce a new required prop on native/components/clearable-text-input.react.ios.js, Flow will not detect an error even though ChatInputBar does not specify this prop. That's because ChatInputBar is typechecking against native/components/clearable-text-input.react.js.

It looks like the preference runs like this: .js > .ios.js > .android.js (maybe determined by order of module.file_ext in .flowconfig)

So doing something like what the React Native repo has with two separate .flowconfigs help catch this sort of type error. But we don't do it currently for typechecking .js vs. .ios.js, so I think we can skip it for now. This diff is a strict improvement either way