Add support for empty dataclass arguments (#3152)

A dataclass with no fields exposed a bug in `extract_dataclass_members`:

```
@dataclass
class Dummy:
  pass
```

The type/return path was inconsistent. This PR fixes the function to
support empty dataclasses, which are useful in unions.

python/CuTeDSL/cutlass/base_dsl/utils/tree_utils.py

@@ -192,7 +192,7 @@ class Leaf:
 # =============================================================================
 
 
-def extract_dataclass_members(x: Any) -> tuple[list[str], list[Any]]:
+def extract_dataclass_members(x: Any) -> tuple[list[str], list[Any], list[Any]]:
     """
     Extract non-method, non-function attributes from a dataclass instance.
 
@@ -200,7 +200,7 @@ def extract_dataclass_members(x: Any) -> tuple[list[str], list[Any]]:
         x: A dataclass instance
 
     Returns:
-        tuple: (field_names, field_values) lists
+        tuple: (field_names, field_values, constexpr_fields) lists
     """
     fields = [field.name for field in dataclasses.fields(x)]
 
@@ -213,7 +213,7 @@ def extract_dataclass_members(x: Any) -> tuple[list[str], list[Any]]:
             )
 
     if not fields:
-        return [], []
+        return [], [], []
 
     # record constexpr fields
     members = []

test/examples/CuTeDSL/test_dataclasses.py

@@ -0,0 +1,72 @@
+# Copyright (c) 2025 - 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: BSD-3-Clause
+
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are met:
+
+# 1. Redistributions of source code must retain the above copyright notice, this
+# list of conditions and the following disclaimer.
+
+# 2. Redistributions in binary form must reproduce the above copyright notice,
+# this list of conditions and the following disclaimer in the documentation
+# and/or other materials provided with the distribution.
+
+# 3. Neither the name of the copyright holder nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+# DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+# SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+from dataclasses import dataclass
+
+import pytest
+import torch
+
+import cutlass
+import cutlass.cute as cute
+from cutlass.cute.runtime import from_dlpack
+
+
+@dataclass
+class A:
+    pass
+
+
+@dataclass
+class B:
+    pass
+
+
+@cute.kernel
+def _test_empty_dataclass_kernel(out: cute.Tensor, tag: A | B):
+    tidx, _, _ = cute.arch.thread_idx()
+    if tidx == 0:
+        match tag:
+            case A():
+                out[0] = 0
+            case B():
+                out[0] = 1
+
+
+@cute.jit
+def _test_empty_dataclass_host(out: cute.Tensor, tag: A | B):
+    _test_empty_dataclass_kernel(out, tag).launch(grid=[1, 1, 1], block=[1, 1, 1])
+
+
+@pytest.mark.parametrize("tag,expected", [(A(), 0), (B(), 1)])
+def test_empty_dataclass_union(tag, expected):
+    out = torch.zeros(1, device="cuda", dtype=torch.int32)
+    out_cute = from_dlpack(out).mark_layout_dynamic()
+    compiled_fn = cute.compile(_test_empty_dataclass_host, out_cute, tag)
+    compiled_fn(out_cute, tag)
+    torch.cuda.synchronize()
+    assert out.item() == expected