Use assert instead for validity check

cpp/include/kvikio/utils.hpp

@@ -15,6 +15,7 @@
  */
 #pragma once
 
+#include <cassert>
 #include <chrono>
 #include <cstring>
 #include <future>
@@ -152,7 +153,7 @@ 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 (!future.valid()) { return true; }
+  assert(future.valid());
 
   // If the future is returned from std::async(std::launch::deferred, ...), its state is always
   // deferred until wait() is called to make it ready.