Regression test for scan_save_mem non-idempotency

tests/scan/rewriting/test_trace.py

@@ -4,12 +4,15 @@
 import pytensor
 import pytensor.tensor as pt
 from pytensor import function, scan, shared
-from pytensor.compile.mode import get_default_mode
+from pytensor.compile.mode import Mode, get_default_mode
 from pytensor.configdefaults import config
 from pytensor.gradient import grad
 from pytensor.graph.basic import Constant
+from pytensor.graph.fg import FunctionGraph
+from pytensor.graph.rewriting.basic import dfs_rewriter
 from pytensor.link.basic import JITLinker
 from pytensor.scan.op import Scan
+from pytensor.scan.rewriting.trace import scan_reduce_trace_prealloc
 from pytensor.scan.utils import until
 from pytensor.tensor.math import dot
 from pytensor.tensor.shape import reshape
@@ -598,6 +601,31 @@ def test_symbolic_stop_not_dropped(self):
         np.testing.assert_allclose(fn(0, 8), ws_ref[3:8])
         np.testing.assert_allclose(fn(0, 10), ws_ref[3:10])
 
+    def test_idempotent(self):
+        """Applying scan_reduce_trace twice must not corrupt negative-step slices."""
+        x0 = scalar("x0")
+        acc = scan(
+            fn=lambda prev: prev + 1,
+            outputs_info=[x0],
+            n_steps=10,
+            return_updates=False,
+            mode=Mode(optimizer=None),
+        )
+        out = acc[7:2:-1]
+
+        rewrite = dfs_rewriter(scan_reduce_trace_prealloc, ignore_newtrees=True)
+        fgraph = FunctionGraph([x0], [out])
+        rewrite.apply(fgraph)
+        rewrite.apply(fgraph)
+
+        f = function(
+            fgraph.inputs,
+            fgraph.outputs[0],
+            accept_inplace=True,
+            mode=Mode(linker="py", optimizer=None),
+        )
+        np.testing.assert_allclose(f(0.0), [8, 7, 6, 5, 4])
+
 
 def test_scan_sit_sot_to_untraced():
     """Test sit_sot to untraced_sit_sot conversion.