Support 0-length struct columns in make_column_from_scalar

cpp/src/column/column_factories.cu

@@ -106,7 +106,6 @@ std::unique_ptr<cudf::column> column_from_scalar_dispatch::operator()<cudf::stru
   cuda::stream_ref stream,
   rmm::device_async_resource_ref mr) const
 {
-  CUDF_EXPECTS(size != 0, "0-length struct column is unsupported.");
   auto& ss  = static_cast<scalar_type_t<cudf::struct_view> const&>(value);
   auto iter = cuda::make_constant_iterator(0);
 

cpp/tests/column/factories_test.cpp

@@ -762,6 +762,24 @@ TEST_F(ColumnFactoryTest, FromStructScalar) { struct_from_scalar(true); }
 
 TEST_F(ColumnFactoryTest, FromStructScalarNull) { struct_from_scalar(false); }
 
+void struct_from_scalar_zero_size(bool is_valid)
+{
+  cudf::test::fixed_width_column_wrapper<int> col0{1};
+  cudf::test::strings_column_wrapper col1{"abc"};
+  cudf::test::lists_column_wrapper<int> col2{{1, 2, 3}};
+
+  std::vector<cudf::column_view> src_children({col0, col1, col2});
+  auto value  = cudf::struct_scalar(src_children, is_valid);
+  auto result = cudf::make_column_from_scalar(value, 0);
+
+  auto const expected = cudf::empty_like(cudf::make_column_from_scalar(value, 1)->view());
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(*result, *expected);
+}
+
+TEST_F(ColumnFactoryTest, FromStructScalarWithZeroSize) { struct_from_scalar_zero_size(true); }
+
+TEST_F(ColumnFactoryTest, FromStructScalarNullWithZeroSize) { struct_from_scalar_zero_size(false); }
+
 TEST_F(ColumnFactoryTest, FromScalarErrors)
 {
   if (cudf::strings::is_large_strings_enabled()) { return; }

python/cudf_polars/tests/expressions/test_struct.py

@@ -132,3 +132,9 @@ def test_struct_literal_field_is_broadcast(
     )
     q = df.select(pl.struct("a", x="c", y=literal).alias("struct").struct.field("y"))
     assert_gpu_result_equal(q, engine=engine)
+
+
+def test_group_by_struct_of_literals_empty_frame(engine: pl.GPUEngine) -> None:
+    df = pl.LazyFrame({"g": [10, 10, 20], "n": [1, 2, 3]}).head(0)
+    q = df.group_by("g").agg(pl.struct(pl.lit(7).alias("lhs"), pl.lit(7).alias("rhs")))
+    assert_gpu_result_equal(q, engine=engine, check_row_order=False)