Fix logic for concatenating Treelite objects (#5387)

Closes #5359

Authors:
  - Philip Hyunsu Cho (https://github.com/hcho3)
  - Dante Gama Dessavre (https://github.com/dantegd)
  - William Hicks (https://github.com/wphicks)

Approvers:
  - William Hicks (https://github.com/wphicks)

URL: #5387

cpp/src/randomforest/randomforest.cu

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019-2022, NVIDIA CORPORATION.
+ * Copyright (c) 2019-2023, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -396,21 +396,25 @@ void compare_concat_forest_to_subforests(ModelHandle concat_tree_handle,
  */
 ModelHandle concatenate_trees(std::vector<ModelHandle> treelite_handles)
 {
-  tl::Model& first_model  = *(tl::Model*)treelite_handles[0];
+  /* TODO(hcho3): Use treelite::ConcatenateModelObjects(),
+     once https://github.com/dmlc/treelite/issues/474 is fixed. */
+  if (treelite_handles.empty()) { return nullptr; }
+  tl::Model& first_model  = *static_cast<tl::Model*>(treelite_handles[0]);
   tl::Model* concat_model = first_model.Dispatch([&treelite_handles](auto& first_model_inner) {
     // first_model_inner is of the concrete type tl::ModelImpl<T, L>
     using model_type   = std::remove_reference_t<decltype(first_model_inner)>;
     auto* concat_model = dynamic_cast<model_type*>(
       tl::Model::Create(first_model_inner.GetThresholdType(), first_model_inner.GetLeafOutputType())
         .release());
     for (std::size_t forest_idx = 0; forest_idx < treelite_handles.size(); forest_idx++) {
-      tl::Model& model  = *(tl::Model*)treelite_handles[forest_idx];
+      tl::Model& model  = *static_cast<tl::Model*>(treelite_handles[forest_idx]);
       auto& model_inner = dynamic_cast<model_type&>(model);
       for (const auto& tree : model_inner.trees) {
         concat_model->trees.push_back(tree.Clone());
       }
     }
     concat_model->num_feature         = first_model_inner.num_feature;
+    concat_model->task_type           = first_model_inner.task_type;
     concat_model->task_param          = first_model_inner.task_param;
     concat_model->average_tree_output = first_model_inner.average_tree_output;
     concat_model->param               = first_model_inner.param;