fix(arrays): return -1 from sorted_last_index_of on empty array

sorted_last_index returns 0 for an empty array (and for any value smaller
than every element), so index becomes -1. The guard only bounded index
from above, so array[-1] was evaluated on an empty list and raised
IndexError. Add the lower-bound check so a negative index is reported as
not found, matching sorted_index_of.

PR.md

@@ -0,0 +1,74 @@
+# Fix `sorted_last_index_of` raising IndexError on empty array
+
+## Problem
+
+`pydash.sorted_last_index_of(array, value)` raises `IndexError` when `array` is
+empty, instead of returning `-1` (the documented "not found" result):
+
+```python
+>>> import pydash
+>>> pydash.sorted_last_index_of([], 10)
+Traceback (most recent call last):
+    ...
+IndexError: list index out of range
+```
+
+The sibling function `sorted_index_of` returns `-1` for the same input, so the
+two are inconsistent.
+
+## Root cause
+
+`src/pydash/arrays.py`, in `sorted_last_index_of`:
+
+```python
+index = sorted_last_index(array, value) - 1
+
+if index < len(array) and array[index] == value:
+    return index
+else:
+    return -1
+```
+
+`sorted_last_index` returns `0` for an empty array (and whenever `value` is
+smaller than every element), so `index` becomes `-1`. The guard only checks the
+upper bound (`index < len(array)`); since `-1 < 0` is `True` for an empty array,
+`array[index]` is evaluated as `array[-1]` on an empty list, raising
+`IndexError`. For a non-empty array the same `index == -1` accidentally avoids a
+wrong result only because `array[-1]` happens not to equal a value smaller than
+`array[0]`; the missing lower-bound check is still a latent correctness issue.
+
+## Fix
+
+Add the lower-bound check so a negative computed index is treated as
+"not found":
+
+```python
+if 0 <= index < len(array) and array[index] == value:
+    return index
+else:
+    return -1
+```
+
+## Test
+
+Extended `test_sorted_last_index_of` in `tests/test_arrays.py` with the
+empty-array case and a value-below-range case:
+
+```python
+([], 10, -1),
+([2, 3, 4], 1, -1),
+```
+
+## Gate
+
+- `python -m pytest tests/test_arrays.py -k test_sorted_last_index_of` -> 4 passed
+- `python -m pytest tests/ --ignore=tests/pytest_mypy_testing` -> 1917 passed
+
+Counterfactual (fix reverted, test kept): the empty-array case fails with
+`IndexError: list index out of range` at `arrays.py` in `sorted_last_index_of`.
+
+## Flags for the account owner
+
+The `tests/pytest_mypy_testing/` suite fails locally with `NameError` (the
+`reveal_type` typing plugin is not resolvable in a bare `pip install -e .`
+environment). These failures are pre-existing and unrelated to this change.

src/pydash/arrays.py

@@ -1794,7 +1794,7 @@ def sorted_last_index_of(
     """
     index = sorted_last_index(array, value) - 1
 
-    if index < len(array) and array[index] == value:
+    if 0 <= index < len(array) and array[index] == value:
         return index
     else:
         return -1

tests/test_arrays.py

@@ -656,6 +656,10 @@ def test_sorted_last_index_by(case, expected):
     [
         ([2, 3, 4, 10, 10], 10, 4),
         ([10, 10, 4, 2, 3], 11, -1),
+        # Empty array must return -1, not raise IndexError.
+        ([], 10, -1),
+        # Value lower than every element also returns -1 without indexing [-1].
+        ([2, 3, 4], 1, -1),
     ],
 )
 def test_sorted_last_index_of(array, value, expected):