fix: handle PandasIndex promotion of dtypes (#11244)

* ffix: handle `PandasIndex` promotion of dtypes

* chore: mypy

Author: ilan-gold

xarray/core/indexes.py

@@ -903,7 +903,8 @@ def join(
         else:
             # how = "inner"
             index = self.index.intersection(other.index)
-
+        if is_allowed_extension_array_dtype(index.dtype):
+            return type(self)(index, self.dim)
         coord_dtype = np.result_type(self.coord_dtype, other.coord_dtype)
         return type(self)(index, self.dim, coord_dtype=coord_dtype)
 

xarray/tests/test_combine.py

@@ -1,10 +1,13 @@
 from __future__ import annotations
 
+import datetime
 import re
 from itertools import product
 
 import numpy as np
+import pandas as pd
 import pytest
+import pytz
 
 from xarray import (
     DataArray,
@@ -1155,6 +1158,22 @@ def test_combine_by_coords_override_order(self) -> None:
         actual = combine_by_coords([x2, x1], compat="override")
         assert_equal(actual["a"], x2["a"])
 
+    def test_combine_by_coords_extension_array(self) -> None:
+        # regression test for https://github.com/pydata/xarray/issues/11235
+        arrs = []
+        expected_vals = []
+        for i in range(2):
+            t = datetime.datetime(2026, 3, 1, hour=i).astimezone(pytz.timezone("UTC"))
+            expected_vals += [t]
+            da = DataArray().expand_dims(
+                time=[t],
+                dummy=[i],
+            )
+            arrs.append(da)
+        expected = pd.array(expected_vals)
+        result = combine_by_coords(arrs, join="outer")
+        pd.testing.assert_extension_array_equal(result["time"].data, expected)
+
 
 class TestCombineMixedObjectsbyCoords:
     def test_combine_by_coords_mixed_unnamed_dataarrays(self):