[Bug]: [PyTorch Frontend] Incorrect translation for aten::slice when step is negative and start/end bounds are omitted

OpenVINO Version

2026.5.0-23104-d903dc909e2 (master)

Operating System

Windows System

Device used for inference

CPU

Framework

None

Model used

No response

Issue description

While exploring src/frontends/pytorch/src/op/slice.cpp, I noticed a // TODO: support default start/end with negative step comment.

Currently, the OpenVINO PyTorch frontend hardcodes the default start bound to 0 and the end bound to INT_MAX for aten::slice when they are omitted (None). However, if the step is negative (e.g., x[::-1]), PyTorch semantics require the start to default to the end of the dimension and end to default to before the beginning of the dimension. Because the sign of step is ignored during translation, converting models that slice with negative steps and omitted bounds produces incorrect results (resulting in RuntimeError: slice step must be positive or empty tensors).

Step-by-step reproduction

Append the following test case to tests/layer_tests/pytorch_tests/test_slice.py:
`
class TestSliceNegativeStepDefaults(PytorchLayerTest):

def _prepare_input(self):
    return (np.array(range(16), np.float32),)
def create_model(self, case):
    if case == "both_default":
        class aten_slice(torch.nn.Module):
            def forward(self, x):
                return x[::-1]
        return aten_slice(), "aten::slice"
    elif case == "start_default":
        class aten_slice(torch.nn.Module):
            def forward(self, x):
                return x[:5:-1]
        return aten_slice(), "aten::slice"
    elif case == "end_default":
        class aten_slice(torch.nn.Module):
            def forward(self, x):
                return x[10::-1]
        return aten_slice(), "aten::slice"
@pytest.mark.parametrize("case", ["both_default", "start_default", "end_default"])
@pytest.mark.nightly
@pytest.mark.precommit
def test_slice_negative_step_defaults(self, ie_device, precision, ir_version, case):
    self._test(*self.create_model(case), ie_device, precision, ir_version)

`

Run the newly added test via pytest:
pytest tests/layer_tests/pytorch_tests/test_slice.py::TestSliceNegativeStepDefaults -v

Relevant log output

DEBUG    openvino.frontend.pytorch.ts_decoder:ts_decoder.py:114 Inlined graph:
graph(%self : __torch__.test_slice.___torch_mangle_44.aten_slice,
      %x.1 : Tensor):
  %2 : int = prim::Constant[value=10]()
  %3 : int = prim::Constant[value=0]()
  %4 : int = prim::Constant[value=-1]() 
  %5 : NoneType = prim::Constant()
  %6 : Tensor = aten::slice(%x.1, %3, %2, %5, %4) 
  return (%6)

RuntimeError: slice step must be positive

=========================== short test summary info ===========================
FAILED tests\layer_tests\pytorch_tests\test_slice.py::TestSliceNegativeStepDefaults::test_slice_negative_step_defaults[ ie_device:CPU - precision:FP32 - case:both_default ]
FAILED tests\layer_tests\pytorch_tests\test_slice.py::TestSliceNegativeStepDefaults::test_slice_negative_step_defaults[ ie_device:CPU - precision:FP32 - case:start_default ]
FAILED tests\layer_tests\pytorch_tests\test_slice.py::TestSliceNegativeStepDefaults::test_slice_negative_step_defaults[ ie_device:CPU - precision:FP32 - case:end_default ]
FAILED tests\layer_tests\pytorch_tests\test_slice.py::TestSliceNegativeStepDefaults::test_slice_negative_step_defaults[ ie_device:GPU - precision:FP32 - case:both_default ]
FAILED tests\layer_tests\pytorch_tests\test_slice.py::TestSliceNegativeStepDefaults::test_slice_negative_step_defaults[ ie_device:GPU - precision:FP32 - case:start_default ]
...
======================= 12 failed, 36 warnings in 3.99s =======================

Issue submission checklist

• I'm reporting an issue. It's not a question.
• I checked the problem with the documentation, FAQ, open issues, Stack Overflow, etc., and have not found a solution.
• There is reproducer code and related data files such as images, videos, models, etc.

Start in src/frontends/pytorch/src/op/slice.cpp at the TODO for default start/end bounds with a negative step, then review the reproducer in tests/layer_tests/pytorch_tests/test_slice.py. Run pytest tests/layer_tests/pytorch_tests/test_slice.py::TestSliceNegativeStepDefaults -v; done means the three negative-step default cases pass without the reported runtime error.