FIX: robust DatetimeIndex.union across DST transitions + flip test to pass (GH#62915)

Author: antznette1

pandas/core/indexes/datetimelike.py

@@ -541,7 +541,14 @@ def _as_range_index(self) -> RangeIndex:
         return RangeIndex(rng)
 
     def _can_range_setop(self, other) -> bool:
-        return isinstance(self.freq, Tick) and isinstance(other.freq, Tick)
+        # Only allow range-based setops when both objects are tick-based AND
+        # not timezone-aware. For tz-aware DatetimeIndex, constant i8 stepping
+        # does not hold across DST transitions in local time, so avoid range path.
+        if not (isinstance(self.freq, Tick) and isinstance(other.freq, Tick)):
+            return False
+        self_tz = getattr(self.dtype, "tz", None)
+        other_tz = getattr(other.dtype, "tz", None)
+        return self_tz is None and other_tz is None
 
     def _wrap_range_setop(self, other, res_i8) -> Self:
         new_freq = None
@@ -726,6 +733,15 @@ def _union(self, other, sort):
             #  that result.freq == self.freq
             return result
         else:
+            # For tz-aware DatetimeIndex, perform union in UTC to avoid
+            # local-time irregularities across DST transitions, then convert back.
+            tz = getattr(self.dtype, "tz", None)
+            if tz is not None:
+                left_utc = self.tz_convert("UTC")
+                right_utc = other.tz_convert("UTC")
+                res_utc = super(type(left_utc), left_utc)._union(right_utc, sort)
+                res = res_utc.tz_convert(tz)
+                return res._with_freq("infer")
             return super()._union(other, sort)._with_freq("infer")
 
     # --------------------------------------------------------------------

pandas/tests/indexes/datetimes/test_setops.py

@@ -61,15 +61,14 @@ def test_union3(self, sort, box):
         tm.assert_index_equal(result, expected)
 
 
-@pytest.mark.xfail(reason="see GH#62915: union across DST boundary", strict=False)
-def test_union_across_dst_boundary_xfail():
+def test_union_across_dst_boundary():
     # US/Eastern DST spring-forward on 2021-03-14 at 02:00
     # (02:00-02:59 local time does not exist)
     tz = "US/Eastern"
     # Left side spans up to the missing hour window
-    left = date_range("2021-03-14 00:00", periods=3, freq="H", tz=tz)
+    left = date_range("2021-03-14 00:00", periods=3, freq="h", tz=tz)
     # right side continues from the first valid post-DST hour
-    right = date_range("2021-03-14 03:00", periods=3, freq="H", tz=tz)
+    right = date_range("2021-03-14 03:00", periods=3, freq="h", tz=tz)
 
     # Expect a union that preserves tz and includes valid hours without duplicates
     expected = DatetimeIndex(
@@ -80,7 +79,7 @@ def test_union_across_dst_boundary_xfail():
             Timestamp("2021-03-14 04:00", tz=tz),
             Timestamp("2021-03-14 05:00", tz=tz),
         ]
-    )
+    ).as_unit(left.unit)
 
     result = left.union(right)
     tm.assert_index_equal(result, expected)