More explicit index concat checking

python/cudf/cudf/core/reshape.py

@@ -291,6 +291,8 @@ def concat(objs, axis=0, join="outer", ignore_index=False, sort=None):
             raise TypeError(
                 "when concatenating indices you must provide ONLY indices"
             )
+        if axis == 1:
+            raise ValueError("cannot concatenate indices across axis 1")
 
     only_series = all(isinstance(o, cudf.Series) for o in objs)
 

python/cudf/cudf/tests/test_concat.py

@@ -1997,3 +1997,38 @@ def test_concat_dict_incorrect_type_index(d):
         match="cannot concatenate a dictionary containing indices",
     ):
         cudf.concat(d, axis=1)
+
+
+@pytest.mark.parametrize(
+    "axis",
+    [
+        0,
+        pytest.param(
+            1,
+            marks=pytest.mark.xfail(
+                reason=("cannot concatenate indices across axis 1")
+            ),
+        ),
+    ],
+)
+@pytest.mark.parametrize(
+    "idx",
+    [
+        [cudf.Index([1, 2, 3])],
+        [cudf.Index([1, 2, 3]), cudf.Index([4, 5, 6])],
+        [
+            cudf.MultiIndex(
+                levels=[[1, 2], ["blue", "red"]],
+                codes=[[0, 0, 1, 1], [1, 0, 1, 0]],
+            )
+        ],
+        [cudf.CategoricalIndex([1, 2, 3])],
+    ],
+)
+def test_concat_index(idx, axis):
+    result = cudf.concat(idx, axis=axis)
+    assert isinstance(result, cudf.Index)
+    with pytest.raises(
+        TypeError, match="only Series and DataFrame objs are valid"
+    ):
+        pd.concat([i.to_pandas() for i in idx], axis=axis)