import org.jspecify.annotations.NullMarked;
import java.util.Set;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ConcurrentHashMap;

@NullMarked
class Foo {
    protected final Set<CompletableFuture<?>> inFlight;

    public Foo() {
        inFlight = ConcurrentHashMap.newKeySet();
    }
}