Android Studio marks R in red with error message “cannot resolve symbol R”, but build succeeds

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Just Click on Build -> Rebuild Project option in your Android Studio.

But in some cases we may additionally need to, use File -> Invalidate Cache... and restart IDE (beside said rebuild, as something of previous “cannot resolve symbol” state may be cached).

We can auto-complete libraries because they are already built, but own project needs to be built first, and only then Android-Studio can see related resources.

#1 Rebuild is required, because Android-Studio does simply rely solely on Gradle’s build-result (without static-analysis support, at least at time of writing, 2022).

#2 Clearing cache may be required, because the previous build-result (before above rebuild) may be indexed, which may make Android Studio ignore latest Gradle build-result
(this is a very good speed optimization, but a very bad Developer-experience as well).

There can sometimes be other reasons too, which is why there are many other answers posted here.

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