From 52100dd3ef04546c2c071cbc5fe2f39dc5768e91 Mon Sep 17 00:00:00 2001
From: Nick Crews <nicholas.b.crews@gmail.com>
Date: Wed, 21 Aug 2024 14:44:12 -0800
Subject: [PATCH] [C++][Go]: support casting nullable fields to non-nullable if
 there are no null values

Fixes https://github.com/apache/arrow/issues/33592
---
 cpp/src/arrow/compute/kernels/scalar_cast_nested.cc |  8 ++++----
 cpp/src/arrow/compute/kernels/scalar_cast_test.cc   | 11 ++++-------
 go/arrow/compute/cast.go                            |  4 ----
 3 files changed, 8 insertions(+), 15 deletions(-)

diff --git a/cpp/src/arrow/compute/kernels/scalar_cast_nested.cc b/cpp/src/arrow/compute/kernels/scalar_cast_nested.cc
index ec5291ef608a3..027d92c7c03f6 100644
--- a/cpp/src/arrow/compute/kernels/scalar_cast_nested.cc
+++ b/cpp/src/arrow/compute/kernels/scalar_cast_nested.cc
@@ -347,6 +347,7 @@ struct CastStruct {
     const int out_field_count = out_type.num_fields();
 
     std::vector<int> fields_to_select(out_field_count, -1);
+    const ArraySpan& in_array = batch[0].array;
 
     int out_field_index = 0;
     for (int in_field_index = 0;
@@ -355,9 +356,9 @@ struct CastStruct {
       const auto& in_field = in_type.field(in_field_index);
       const auto& out_field = out_type.field(out_field_index);
       if (in_field->name() == out_field->name()) {
-        if (in_field->nullable() && !out_field->nullable()) {
-          return Status::TypeError("cannot cast nullable field to non-nullable field: ",
-                                   in_type.ToString(), " ", out_type.ToString());
+        if (!out_field->nullable() && in_array.null_count > 0) {
+          return Status::TypeError("casting field ", in_field->name(),
+                      " with nulls to non-nullable type", out_type.ToString());
         }
         fields_to_select[out_field_index++] = in_field_index;
       }
@@ -369,7 +370,6 @@ struct CastStruct {
           in_type.ToString(), " output fields: ", out_type.ToString());
     }
 
-    const ArraySpan& in_array = batch[0].array;
     ArrayData* out_array = out->array_data().get();
 
     if (in_array.buffers[0].data != nullptr) {
diff --git a/cpp/src/arrow/compute/kernels/scalar_cast_test.cc b/cpp/src/arrow/compute/kernels/scalar_cast_test.cc
index 140789e59665b..c126da8244431 100644
--- a/cpp/src/arrow/compute/kernels/scalar_cast_test.cc
+++ b/cpp/src/arrow/compute/kernels/scalar_cast_test.cc
@@ -2855,7 +2855,7 @@ TEST(Cast, StructToDifferentNullabilityStruct) {
     CheckCast(src_non_nullable, dest3_nullable);
   }
   {
-    // But NOT OK to go from nullable to non-nullable...
+    // But when going from nullable to non-nullable, all data must be non-null...
     std::vector<std::shared_ptr<Field>> fields_src_nullable = {
         std::make_shared<Field>("a", int8(), true),
         std::make_shared<Field>("b", int8(), true),
@@ -2877,7 +2877,7 @@ TEST(Cast, StructToDifferentNullabilityStruct) {
     const auto options1_non_nullable = CastOptions::Safe(dest1_non_nullable);
     EXPECT_RAISES_WITH_MESSAGE_THAT(
         TypeError,
-        ::testing::HasSubstr("cannot cast nullable field to non-nullable field"),
+        ::testing::HasSubstr("casting field a with nulls to non-nullable type int not null"),
         Cast(src_nullable, options1_non_nullable));
 
     std::vector<std::shared_ptr<Field>> fields_dest2_non_nullable = {
@@ -2887,17 +2887,14 @@ TEST(Cast, StructToDifferentNullabilityStruct) {
     const auto options2_non_nullable = CastOptions::Safe(dest2_non_nullable);
     EXPECT_RAISES_WITH_MESSAGE_THAT(
         TypeError,
-        ::testing::HasSubstr("cannot cast nullable field to non-nullable field"),
+        ::testing::HasSubstr("casting field a with nulls to non-nullable type int not null"),
         Cast(src_nullable, options2_non_nullable));
 
     std::vector<std::shared_ptr<Field>> fields_dest3_non_nullable = {
         std::make_shared<Field>("c", int64(), false)};
     const auto dest3_non_nullable = arrow::struct_(fields_dest3_non_nullable);
     const auto options3_non_nullable = CastOptions::Safe(dest3_non_nullable);
-    EXPECT_RAISES_WITH_MESSAGE_THAT(
-        TypeError,
-        ::testing::HasSubstr("cannot cast nullable field to non-nullable field"),
-        Cast(src_nullable, options3_non_nullable));
+    CheckCast(src_nullable, dest3_non_nullable, options3_non_nullable);
   }
 }
 
diff --git a/go/arrow/compute/cast.go b/go/arrow/compute/cast.go
index 6ef6fdddd16ff..02ad2595225f9 100644
--- a/go/arrow/compute/cast.go
+++ b/go/arrow/compute/cast.go
@@ -280,10 +280,6 @@ func CastStruct(ctx *exec.KernelCtx, batch *exec.ExecSpan, out *exec.ExecResult)
 		inField := inType.Field(inFieldIndex)
 		outField := outType.Field(outFieldIndex)
 		if inField.Name == outField.Name {
-			if inField.Nullable && !outField.Nullable {
-				return fmt.Errorf("%w: cannot cast nullable field to non-nullable field: %s %s",
-					arrow.ErrType, inType, outType)
-			}
 			fieldsToSelect[outFieldIndex] = inFieldIndex
 			outFieldIndex++
 		}