BUG: DatetimeIndex.snap incorrectly setting freq (#31188)

Author: jbrockmendel

pandas/core/indexes/datetimes.py

@@ -486,8 +486,8 @@ def snap(self, freq="S"):
                     s = t1
             snapped[i] = s
 
-        # we know it conforms; skip check
-        return DatetimeIndex._simple_new(snapped, name=self.name, tz=self.tz, freq=freq)
+        dta = DatetimeArray(snapped, dtype=self.dtype)
+        return DatetimeIndex._simple_new(dta, name=self.name)
 
     def _parsed_string_to_bounds(self, reso, parsed):
         """

pandas/tests/indexes/datetimes/test_snap.py

@@ -28,10 +28,14 @@ def test_dti_snap(name, tz):
     expected = expected.repeat([3, 4])
     tm.assert_index_equal(result, expected)
     assert result.tz == expected.tz
+    assert result.freq is None
+    assert expected.freq is None
 
     result = dti.snap(freq="B")
 
     expected = date_range("1/1/2002", "1/7/2002", name=name, tz=tz, freq="b")
     expected = expected.repeat([1, 1, 1, 2, 2])
     tm.assert_index_equal(result, expected)
     assert result.tz == expected.tz
+    assert result.freq is None
+    assert expected.freq is None