Close gh-1779

`diff` now raises an exception for negative `n` as expected

Author: ndgrigorian

dpctl/tensor/_utility_functions.py

@@ -453,7 +453,8 @@ def diff(x, /, *, axis=-1, n=1, prepend=None, append=None):
     x_nd = x.ndim
     axis = normalize_axis_index(operator.index(axis), x_nd)
     n = operator.index(n)
-
+    if n < 0:
+        raise ValueError(f"`n` must be positive, got {n}")
     arr = _concat_diff_input(x, axis, prepend, append)
     if n == 0:
         return arr

dpctl/tests/test_tensor_diff.py

@@ -313,3 +313,17 @@ def test_diff_input_validation():
         dpt.diff,
         bad_in,
     )
+
+
+def test_diff_positive_order():
+    get_queue_or_skip()
+
+    x = dpt.ones(1, dtype="i4")
+    n = -1
+    assert_raises_regex(
+        ValueError,
+        ".*must be positive.*",
+        dpt.diff,
+        x,
+        n=n,
+    )