Virtual thread API migration failures from JDK 19/20 preview to JDK 21 final
Goal: diagnose why code that compiled and ran with the preview virtual thread API in JDK 19 or JDK 20 fails when compiled or executed on JDK 21, where virtual threads are finalized and the API has changed. Constraints: the code may use preview‑only imports (jdk.incubator.concurrent), preview‑only constructors, and custom scheduler implementations that relied
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