Is this a Java bug or am I misusing Java (preview)'s structured task scope?

So I have this code excerpt (a minimal reproduction, from a much larger project, and blocking issue). It leverages Java 22 (preview)'s structured task scope in combo with virtual threads:

playground.java

void main() throws InterruptedException {
  final var NAME = ScopedValue.<String>newInstance();
  try (var ts = new StructuredTaskScope<>()) {
    ScopedValue.runWhere(NAME, "haha", () -> {
      ts.fork(() -> {
   // ^^^
   // java.util.concurrent.StructureViolationException: Scoped value bindings have changed
        return null;
      });
    });
    ts.join();
  }
}

Using Java 22, you can run it with java --enable-preview --source 22 playground.java.

You are using the structured task scope wrong.

The JavaDoc for StructuredTaskScope.fork() states:

StructureViolationExceptionPREVIEW - if the current scoped value bindings are not the same as when the task scope was created

Your code changes the scoped value bindings by calling ScopedValue.runWhere(NAME, "haha", () -> {}); after the task scope was created.

You should probably structure your code like this:

void main() throws InterruptedException {
  final var NAME = ScopedValue.<String>newInstance();
  ScopedValue.runWhere(NAME, "haha", () -> {
    try (var ts = new StructuredTaskScope<>()) {
      ts.fork(() -> {
        return null;
      });
    });
    ts.join();
  }
}

Or you can create a new task scope after changing the scoped value:

void main() throws InterruptedException {
  final var NAME = ScopedValue.<String>newInstance();
  try (var ts = new StructuredTaskScope<>()) {
    ScopedValue.runWhere(NAME, "haha", () -> {
      try (var ts2 = new StructuredTaskScope<>()) {
        ts2.fork(() -> {
          return null;
        });
        ts2.join();
      }
    });
    ts.join();
  }
}