GH-37072: [C++] MakeArrayOfNulls should respect Field::nullable

cpp/src/arrow/array/array_run_end_test.cc

@@ -542,9 +542,7 @@ TEST_P(TestRunEndEncodedArray, Validate) {
   BitmapFromVector<bool>({true, false}, &null_bitmap);
   has_null_buffer->data()->buffers[0] = null_bitmap;
   EXPECT_RAISES_WITH_MESSAGE_THAT(
-      Invalid,
-      ::testing::HasSubstr(
-          std::string("Invalid: Run end encoded array should not have a null bitmap.")),
+      Invalid, ::testing::HasSubstr(std::string("should not have a null bitmap")),
       has_null_buffer->Validate());
 
   auto too_many_children = MakeArray(good_array->data()->Copy());
@@ -570,7 +568,7 @@ TEST_P(TestRunEndEncodedArray, Validate) {
       values_nullptr->Validate());
 
   auto run_ends_string = MakeArray(good_array->data()->Copy());
-  run_ends_string->data()->child_data[0] = values->data();
+  run_ends_string->data()->child_data[0] = MakeEmptyArray(utf8()).ValueOrDie()->data();
   EXPECT_RAISES_WITH_MESSAGE_THAT(
       Invalid,
       ::testing::HasSubstr(
@@ -596,10 +594,9 @@ TEST_P(TestRunEndEncodedArray, Validate) {
     malformed_array->data()->buffers.emplace_back(NULLPTR);
     EXPECT_RAISES_WITH_MESSAGE_THAT(
         Invalid,
-        ::testing::HasSubstr(
-            std::string(
-                "Invalid: Run ends array invalid: Expected 2 buffers in array of type ") +
-            run_end_type->ToString() + ", got 3"),
+        ::testing::HasSubstr(std::string("Invalid: Run ends array invalid: Invalid: "
+                                         "Expected 2 buffers in array of type ") +
+                             run_end_type->ToString() + ", got 3"),
         run_ends_malformed->Validate());
   }
 
@@ -610,8 +607,9 @@ TEST_P(TestRunEndEncodedArray, Validate) {
     malformed_array->data()->buffers.emplace_back(NULLPTR);
     EXPECT_RAISES_WITH_MESSAGE_THAT(
         Invalid,
-        ::testing::HasSubstr("Invalid: Values array invalid: Expected 2 buffers in array "
-                             "of type int32, got 3"),
+        ::testing::HasSubstr(
+            "Invalid: Values array invalid: Invalid: Expected 2 buffers in array "
+            "of type int32, got 3"),
         values_malformed->Validate());
   }
 

cpp/src/arrow/array/array_test.cc

@@ -464,6 +464,7 @@ static std::vector<std::shared_ptr<DataType>> TestArrayUtilitiesAgainstTheseType
       list_view(utf8()),
       large_list_view(utf8()),
       dictionary(int32(), utf8()),
+      struct_({}),
       struct_({field("a", utf8()), field("b", int32())}),
       sparse_union(union_fields1, union_type_codes),
       sparse_union(union_fields2, union_type_codes),
@@ -477,6 +478,7 @@ static std::vector<std::shared_ptr<DataType>> TestArrayUtilitiesAgainstTheseType
 
 TEST_F(TestArray, TestMakeArrayOfNull) {
   for (int64_t length : {0, 1, 16, 133}) {
+    ARROW_SCOPED_TRACE("length = ", length);
     for (auto type : TestArrayUtilitiesAgainstTheseTypes()) {
       ARROW_SCOPED_TRACE("type = ", type->ToString());
       ASSERT_OK_AND_ASSIGN(auto array, MakeArrayOfNull(type, length));
@@ -506,6 +508,29 @@ TEST_F(TestArray, TestMakeArrayOfNull) {
       }
     }
   }
+
+  for (int64_t length : {0, 16}) {
+    ARROW_SCOPED_TRACE("length = ", length, " (required fields)");
+
+    auto req = [](auto type) { return field("", std::move(type), /*nullable=*/false); };
+
+    // union with no nullable fields cannot represent a null
+    ASSERT_RAISES(TypeError, MakeArrayOfNull(dense_union({req(int8())}), length));
+    // run end encoded with non nullable child cannot represent a null
+    // (not directly constructible, but not invalid per Columnar.rst)
+    auto ree = run_end_encoded(int16(), utf8());
+    const_cast<FieldVector&>(ree->fields())[1] = req(utf8());
+    ASSERT_RAISES(TypeError, MakeArrayOfNull(ree, length));
+
+    // struct with no nullable fields has a top level bitmap and can mask them
+    ASSERT_OK_AND_ASSIGN(auto s, MakeArrayOfNull(struct_({req(int8())}), length));
+    ASSERT_OK(s->ValidateFull());
+
+    // dictionary with non-nullable indices can use a 1-long dict of null
+    ASSERT_OK_AND_ASSIGN(
+        s, MakeArrayOfNull(struct_({req(dictionary(int8(), int8()))}), length));
+    ASSERT_OK(s->ValidateFull());
+  }
 }
 
 TEST_F(TestArray, TestMakeArrayOfNullUnion) {