Take deferred future into account for status query

cpp/include/kvikio/utils.hpp

@@ -152,6 +152,17 @@ std::tuple<void*, std::size_t, std::size_t> get_alloc_info(const void* devPtr,
 template <typename T>
 bool is_future_done(const T& future)
 {
+  // If the future does not refer to a valid shared state.
+  if (!future.valid()) { return true; }
+
+  // If the future is returned from std::async(std::launch::deferred), its
+  // state is always deferred until wait() is called.
+  if (future.wait_for(std::chrono::seconds(0)) == std::future_status::deferred) {
+    future.wait();
+    return true;
+  }
+
+  // If the future is returned from std::async(std::launch:async).
   return future.wait_for(std::chrono::seconds(0)) != std::future_status::timeout;
 }