Fix astimezone conversion with dateutil timezones

src/pendulum/datetime.py

@@ -1273,7 +1273,19 @@ def combine(
         return cls.instance(datetime.datetime.combine(date, time), tz=tzinfo)
 
     def astimezone(self, tz: datetime.tzinfo | None = None) -> Self:
-        dt = super().astimezone(tz)
+        # Timezone implementations may do arithmetic in fromutc(), which must
+        # use datetime's rules rather than Pendulum's duration arithmetic.
+        dt = datetime.datetime(
+            self.year,
+            self.month,
+            self.day,
+            self.hour,
+            self.minute,
+            self.second,
+            self.microsecond,
+            tzinfo=self.tzinfo,
+            fold=self.fold,
+        ).astimezone(tz)
 
         return self.__class__(
             dt.year,

tests/datetime/test_timezone.py

@@ -1,5 +1,14 @@
 from __future__ import annotations
 
+from datetime import datetime
+from datetime import timedelta
+from datetime import timezone
+from zoneinfo import ZoneInfo
+
+import pytest
+
+from dateutil import tz
+
 import pendulum
 
 from tests.conftest import assert_datetime
@@ -36,3 +45,54 @@ def test_astimezone():
     d = d.astimezone(pendulum.timezone("Europe/Paris"))
     assert d.timezone_name == "Europe/Paris"
     assert_datetime(d, now.year, now.month, now.day, now.hour + 1, now.minute)
+
+
+class CustomDateTime(pendulum.DateTime):
+    pass
+
+
+@pytest.mark.parametrize("datetime_class", [pendulum.DateTime, CustomDateTime])
+@pytest.mark.parametrize(
+    "target",
+    [
+        pytest.param(tz.gettz("Etc/GMT+6"), id="dateutil-fixed-zone"),
+        pytest.param(tz.gettz("Europe/Paris"), id="dateutil-dst-zone"),
+        pytest.param(tz.tzoffset("offset", 19800), id="dateutil-offset"),
+        pytest.param(timezone(timedelta(hours=5, minutes=30)), id="builtin"),
+        pytest.param(ZoneInfo("Europe/Paris"), id="zoneinfo"),
+        pytest.param(pendulum.timezone("Europe/Paris"), id="pendulum"),
+        pytest.param(None, id="local"),
+    ],
+)
+@pytest.mark.parametrize(
+    "source",
+    [
+        datetime(2024, 1, 15, 18, 15, 34, 123456, tzinfo=timezone.utc),
+        datetime(2024, 7, 15, 18, 15, 34, 123456, tzinfo=timezone.utc),
+        datetime(2024, 10, 27, 0, 30, tzinfo=timezone.utc),
+        datetime(2024, 10, 27, 1, 30, tzinfo=timezone.utc),
+        datetime(2024, 10, 27, 2, 30, tzinfo=ZoneInfo("Europe/Paris"), fold=1),
+        datetime(2024, 1, 15, 18, 15, 34, 123456),
+    ],
+)
+def test_astimezone_matches_datetime(datetime_class, target, source):
+    value = datetime_class(
+        source.year,
+        source.month,
+        source.day,
+        source.hour,
+        source.minute,
+        source.second,
+        source.microsecond,
+        tzinfo=source.tzinfo,
+        fold=source.fold,
+    )
+
+    expected = source.astimezone(target)
+    actual = value.astimezone(target)
+
+    assert type(actual) is datetime_class
+    assert actual.tzinfo == expected.tzinfo
+    assert actual.isoformat() == expected.isoformat()
+    assert actual.timestamp() == expected.timestamp()
+    assert actual.fold == expected.fold