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Make is_supported_cast fail for variable-width inputs #23613

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Make is_supported_cast fail for variable-width inputs #23613

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48f80e3
Make is_supported_cast fail for variable-width inputs
JohnZed Aug 10, 2026
dfb999b
[pre-commit.ci] auto code formatting
pre-commit-ci[bot] Aug 10, 2026
e86fcff
Fix cast tests header comment
JohnZed Aug 10, 2026
7edb940
Expand to test multiple unsupported input types in IsSupportedCase test
JohnZed Aug 14, 2026
8914db2
Update cast header docs to reflect variable width inputs unsupported
JohnZed Aug 14, 2026
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2 changes: 1 addition & 1 deletion cpp/include/cudf/unary.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -117,7 +117,7 @@ std::unique_ptr<cudf::column> is_valid(
* @param mr Device memory resource used to allocate the returned column's device memory
*
* @returns Column of same size as `input` containing result of the cast operation
* @throw cudf::logic_error if `out_type` is not a fixed-width type
* @throw cudf::logic_error if `input` or `out_type` is not a fixed-width type
*/
std::unique_ptr<column> cast(
column_view const& input,
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4 changes: 2 additions & 2 deletions cpp/src/unary/cast_ops.cu
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
/*
* SPDX-FileCopyrightText: Copyright (c) 2019-2026, NVIDIA CORPORATION.
* SPDX-FileCopyrightText: Copyright (c) 2019-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
* SPDX-License-Identifier: Apache-2.0
*/

Expand Down Expand Up @@ -118,7 +118,7 @@ struct fixed_point_unary_cast {
template <typename From, typename To>
constexpr inline auto is_supported_non_fixed_point_cast()
{
return cudf::is_fixed_width<To>() &&
return cudf::is_fixed_width<From>() && cudf::is_fixed_width<To>() &&

@igorpeshansky igorpeshansky Aug 12, 2026

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Note

Your PR description says that you're matching the "cudf cast documentation", but the cast will, in fact, fail for those non-fixed-width types, so you're in fact matching the cast implementation. Might be worth a tweak to the PR description…

[Optional] Might also be worth it to update the is_supported_cast function doc in unary.hpp (lines 129-136)…

@JohnZed JohnZed Aug 14, 2026

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@igorpeshansky - agreed about expanding the test above and took your suggestion there. Changed the PR description too. Thanks!
Not sure I see a change to the header docs for is_supported_cast as necessary here though. I feel like the current description of "Check if a cast between two datatypes is supported." is a good one - nice and concise, and I don't know that it needs to spell out all the possible cast failure paths.
The documentation of cast in unary.hpp was a little funny though - it mentioned failing on variable width output_type, but not variable-width inputs. So I updated that one.

// Disallow fixed_point here (requires different specialization)
!(cudf::is_fixed_point<From>() || cudf::is_fixed_point<To>()) &&
// Disallow conversions between timestamps and numeric
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11 changes: 10 additions & 1 deletion cpp/tests/unary/cast_tests.cpp
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
/*
* SPDX-FileCopyrightText: Copyright (c) 2020-2026, NVIDIA CORPORATION.
* SPDX-FileCopyrightText: Copyright (c) 2020-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
* SPDX-License-Identifier: Apache-2.0
*/

Expand Down Expand Up @@ -201,6 +201,15 @@ inline auto make_data_type()
return cudf::data_type{cudf::type_to_id<T>()};
}

TEST(IsSupportedCast, UnsupportedTypes)
{
auto const to_int32 = cudf::data_type{cudf::type_id::INT32};
EXPECT_FALSE(cudf::is_supported_cast(cudf::data_type{cudf::type_id::STRING}, to_int32));
EXPECT_FALSE(cudf::is_supported_cast(cudf::data_type{cudf::type_id::LIST}, to_int32));
EXPECT_FALSE(cudf::is_supported_cast(cudf::data_type{cudf::type_id::STRUCT}, to_int32));
EXPECT_FALSE(cudf::is_supported_cast(cudf::data_type{cudf::type_id::DICTIONARY32}, to_int32));
}

struct CastTimestampsSimple : public cudf::test::BaseFixture {};

TEST_F(CastTimestampsSimple, IsIdempotent)
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