From c10966cc3847ca9837ddc7ce5df9c4d9b7c743d8 Mon Sep 17 00:00:00 2001
From: Alfred Xu <lovedreamf@gmail.com>
Date: Thu, 2 Dec 2021 18:48:03 +0800
Subject: [PATCH] Fix make_empty_scalar_like on list_type (#9759)

Fixes #9758

In `make_empty_scalar_like`, we create list scalar with the list column itself, which is wrong. The correct way is with the child of list column.

Authors:
  - Alfred Xu (https://github.com/sperlingxx)

Approvers:
  - Nghia Truong (https://github.com/ttnghia)
  - Devavret Makkar (https://github.com/devavret)

URL: https://github.com/rapidsai/cudf/pull/9759
---
 cpp/src/scalar/scalar_factories.cpp      | 7 +++++--
 cpp/tests/reductions/reduction_tests.cpp | 8 ++++++--
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/cpp/src/scalar/scalar_factories.cpp b/cpp/src/scalar/scalar_factories.cpp
index d2876435780..c18b57d220f 100644
--- a/cpp/src/scalar/scalar_factories.cpp
+++ b/cpp/src/scalar/scalar_factories.cpp
@@ -21,6 +21,7 @@
 #include <cudf/utilities/type_dispatcher.hpp>
 
 #include <cudf/detail/copy.hpp>
+#include <cudf/lists/lists_column_view.hpp>
 #include <rmm/cuda_stream_view.hpp>
 
 namespace cudf {
@@ -184,10 +185,12 @@ std::unique_ptr<scalar> make_empty_scalar_like(column_view const& column,
 {
   std::unique_ptr<scalar> result;
   switch (column.type().id()) {
-    case type_id::LIST:
-      result = make_list_scalar(empty_like(column)->view(), stream, mr);
+    case type_id::LIST: {
+      auto const empty_child = empty_like(lists_column_view(column).child());
+      result                 = make_list_scalar(empty_child->view(), stream, mr);
       result->set_valid_async(false, stream);
       break;
+    }
     case type_id::STRUCT:
       // The input column must have at least 1 row to extract a scalar (row) from it.
       result = detail::get_element(column, 0, stream, mr);
diff --git a/cpp/tests/reductions/reduction_tests.cpp b/cpp/tests/reductions/reduction_tests.cpp
index d8ee8f9d08d..e138cd6f68e 100644
--- a/cpp/tests/reductions/reduction_tests.cpp
+++ b/cpp/tests/reductions/reduction_tests.cpp
@@ -1961,7 +1961,11 @@ struct ListReductionTest : public cudf::test::BaseFixture {
         cudf::reduce(input_data, agg, cudf::data_type(cudf::type_id::LIST));
       auto list_result = dynamic_cast<cudf::list_scalar*>(result.get());
       EXPECT_EQ(is_valid, list_result->is_valid());
-      if (is_valid) { CUDF_TEST_EXPECT_COLUMNS_EQUAL(expected_value, list_result->view()); }
+      if (is_valid) {
+        CUDF_TEST_EXPECT_COLUMNS_EQUAL(expected_value, list_result->view());
+      } else {
+        CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(expected_value, list_result->view());
+      }
     };
 
     if (succeeded_condition) {
@@ -2047,7 +2051,7 @@ TEST_F(ListReductionTest, NonValidListReductionNthElement)
 
   // test against empty input
   this->reduction_test(LCW{},
-                       ElementCol{{0}, {0}},  // expected_value,
+                       ElementCol{},  // expected_value,
                        true,
                        false,
                        cudf::make_nth_element_aggregation(0, cudf::null_policy::INCLUDE));