BUG: `array_api.argsort(descending=True)` respects relative sort order (#20788)

* BUG: `array_api.argsort(descending=True)` respects relative order

* Regression test for stable descending `array_api.argsort()`

1 files changed, 23 insertions, 0 deletions

diff --git a/numpy/array_api/tests/test_sorting_functions.py b/numpy/array_api/tests/test_sorting_functions.py
new file mode 100644
index 000000000..9848bbfeb
--- /dev/null
+++ b/numpy/array_api/tests/test_sorting_functions.py
@@ -0,0 +1,23 @@
+import pytest
+
+from numpy import array_api as xp
+
+
+@pytest.mark.parametrize(
+    "obj, axis, expected",
+    [
+        ([0, 0], -1, [0, 1]),
+        ([0, 1, 0], -1, [1, 0, 2]),
+        ([[0, 1], [1, 1]], 0, [[1, 0], [0, 1]]),
+        ([[0, 1], [1, 1]], 1, [[1, 0], [0, 1]]),
+    ],
+)
+def test_stable_desc_argsort(obj, axis, expected):
+    """
+    Indices respect relative order of a descending stable-sort
+
+    See https://github.com/numpy/numpy/issues/20778
+    """
+    x = xp.asarray(obj)
+    out = xp.argsort(x, axis=axis, stable=True, descending=True)
+    assert xp.all(out == xp.asarray(expected))