TST: fix Numpy Nightly CI failure in test_dti_union_mixed (pandas-dev#65848)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: jbrockmendel

pandas/compat/numpy/__init__.py

@@ -13,6 +13,7 @@
 np_version_gt2_2 = _nlv >= Version("2.2.0")
 np_version_gt2_3 = _nlv >= Version("2.3.0")
 np_version_gt2_5 = _nlv >= Version("2.5.0")
+np_version_gt2_6 = _nlv >= Version("2.6.0.dev0")
 is_numpy_dev = _nlv.dev is not None
 _min_numpy_ver = "1.26.0"
 

pandas/tests/indexes/datetimes/test_setops.py

@@ -7,6 +7,7 @@
 import numpy as np
 import pytest
 
+from pandas.compat.numpy import np_version_gt2_6
 import pandas.util._test_decorators as td
 
 import pandas as pd
@@ -527,7 +528,11 @@ def test_dti_union_mixed(self):
         # GH#21671
         rng = DatetimeIndex([Timestamp("2011-01-01"), pd.NaT])
         rng2 = DatetimeIndex(["2012-01-01", "2012-01-02"], tz="Asia/Tokyo")
-        result = rng.union(rng2)
+        # numpy>=2.6 object-dtype sort compares the tz-naive and tz-aware
+        #  Timestamps directly, so safe_sort fails and union warns
+        warn = RuntimeWarning if np_version_gt2_6 else None
+        with tm.assert_produces_warning(warn, match="sort order is undefined"):
+            result = rng.union(rng2)
         expected = Index(
             [
                 Timestamp("2011-01-01"),