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kotlinlang

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Hi everyone. maybe a bug of kotlin-gradle-plugin found.
*First compilation after clean build fallback to rebuild when build-cache enabled*
<https://youtrack.jetbrains.com/issue/KT-39656>

Steps
1. Enable gradle <https://docs.gradle.org/current/userguide/build_cache.html|Build Cache>
2. Run `./gradlew clean assembleDebug`
3. Modify .kt files that may cause incremental compilation
What I expect: Only out-of-date files will be compiled
What I got:  All files get compiled
Probable cause:

see the three screenshots of kotlin-gradle-plugin code below

1. KotlinCompile is a `@CacheableTask`, so gradle will store it’s `@Output` files to make build cache happens.
2. `taskBuildDirectory` is marked as `@LocalState`, which will be removed when the task is loaded from cache.
3. when first incremental happens, kotlin check if `taskBuildDirectory` exists. If not, `chagedFiles` will be assigned to `ChangedFiles.UnKnown`, which cause kotlin fallback to rebuild. It will waste a plenty of meaningless time.
