From 4e6e96985dbfaa7938ef0ea6fff8eb9bd08407c2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E6=9E=97SO?=
 <142557582+Linxiushen@users.noreply.github.com>
Date: Mon, 28 Sep 2026 01:50:12 +0800
Subject: [PATCH] Fix last-element indexing mixed with slices or scalar indices

---
 onnxscript/_internal/converter.py    |  4 +-
 onnxscript/_internal/slicing_test.py | 82 ++++++++++++++++++++++++++++
 onnxscript/tensor.py                 |  4 +-
 3 files changed, 87 insertions(+), 3 deletions(-)
 create mode 100644 onnxscript/_internal/slicing_test.py

diff --git a/onnxscript/_internal/converter.py b/onnxscript/_internal/converter.py
index c2f6b0fb63..b3cb8a7b8c 100644
--- a/onnxscript/_internal/converter.py
+++ b/onnxscript/_internal/converter.py
@@ -775,7 +775,6 @@ def translate_slice(slice_expr: ast.Slice) -> tuple[ir.Value, ir.Value, ir.Value
             squeezed_axes = []
             for axis, expr in scalar_indices:
                 # Treat a scalar index i as slice "i:i+1:1", but squeeze the axis finally.
-                # TODO: handle negative i
                 index = self._eval_constant_expr(expr)
                 squeezed_axes.append(axis)
                 kwargs = dict(
@@ -784,7 +783,8 @@ def translate_slice(slice_expr: ast.Slice) -> tuple[ir.Value, ir.Value, ir.Value
                 )
                 element = ast.Slice(
                     ast.Constant(index, **kwargs),
-                    ast.Constant(index + 1, **kwargs),
+                    # -1 selects the last element, so its stop must reach the axis end.
+                    ast.Constant(maxint if index == -1 else index + 1, **kwargs),
                     ast.Constant(1, **kwargs),
                 )
                 sliced_indices.append((axis, element))
diff --git a/onnxscript/_internal/slicing_test.py b/onnxscript/_internal/slicing_test.py
new file mode 100644
index 0000000000..16ad8a63f9
--- /dev/null
+++ b/onnxscript/_internal/slicing_test.py
@@ -0,0 +1,82 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+"""Parity tests for scalar indexing combined with slices."""
+
+import itertools
+import unittest
+
+import numpy as np
+import onnx
+import onnxruntime as ort
+import parameterized
+
+from onnxscript import INT64, evaluator, opset15, script
+
+
+@script(default_opset=opset15)
+def last_row_slice(x: INT64["N", "M"]) -> INT64["K"]:  # noqa: F821
+    return x[-1, :2]
+
+
+@script(default_opset=opset15)
+def last_column_slice(x: INT64["N", "M"]) -> INT64["K"]:  # noqa: F821
+    return x[:2, -1]
+
+
+@script(default_opset=opset15)
+def last_element(x: INT64["N", "M"]) -> INT64:  # noqa: F821
+    return x[-1, -1]
+
+
+@script(default_opset=opset15)
+def penultimate_row_last_column(x: INT64["N", "M"]) -> INT64:  # noqa: F821
+    return x[-2, -1]
+
+
+@script(default_opset=opset15)
+def reversed_last_row(x: INT64["N", "M"]) -> INT64["M"]:  # noqa: F821
+    return x[-1, ::-1]
+
+
+@script(default_opset=opset15)
+def first_row_slice(x: INT64["N", "M"]) -> INT64["K"]:  # noqa: F821
+    return x[0, :2]
+
+
+class TestScalarSlicing(unittest.TestCase):
+    @parameterized.parameterized.expand(
+        itertools.product(
+            [
+                (last_row_slice, (-1, slice(None, 2))),
+                (last_column_slice, (slice(None, 2), -1)),
+                (last_element, (-1, -1)),
+                (penultimate_row_last_column, (-2, -1)),
+                (reversed_last_row, (-1, slice(None, None, -1))),
+                (first_row_slice, (0, slice(None, 2))),
+            ],
+            [(4, 3), (2, 5)],
+            ["eager", "onnxruntime"],
+        )
+    )
+    def test_matches_numpy(self, function_and_index, shape, mode):
+        function, index = function_and_index
+        data = np.arange(np.prod(shape), dtype=np.int64).reshape(shape)
+        if mode == "eager":
+            with evaluator.default_as(evaluator.OnnxReferenceRuntimeEvaluator()):
+                actual = function(data)
+        else:
+            model = function.to_model_proto(ir_version=9)
+            onnx.checker.check_model(model)
+            options = ort.SessionOptions()
+            options.intra_op_num_threads = 1
+            options.inter_op_num_threads = 1
+            session = ort.InferenceSession(
+                model.SerializeToString(), options, providers=["CPUExecutionProvider"]
+            )
+            actual = session.run(None, {"x": data})[0]
+        np.testing.assert_array_equal(actual, data[index])
+        self.assertEqual(actual.shape, data[index].shape)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/onnxscript/tensor.py b/onnxscript/tensor.py
index 0c70cc57be..91090abaf6 100644
--- a/onnxscript/tensor.py
+++ b/onnxscript/tensor.py
@@ -118,7 +118,9 @@ def __getitem__(self, index):
                     )
             elif isinstance(s, Tensor):
                 if s.is_scalar:
-                    scalar_indices.append([s, s + 1, axis_, 1])
+                    # A stop of zero would make the last-element slice empty.
+                    stop = shape[axis_] if int(s) == -1 else s + 1
+                    scalar_indices.append([s, stop, axis_, 1])
                     to_squeeze.append(axis_)
                 else:
                     non_scalar_indices.append((axis_, s))