BUG: Collision between equivalent frequencies 'QS-…

doc/source/whatsnew/v3.0.0.rst

@@ -654,6 +654,7 @@ Datetimelike
 - Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` returning ``False`` on double-digit frequencies (:issue:`58523`)
 - Bug in :meth:`DatetimeIndex.union` and :meth:`DatetimeIndex.intersection` when ``unit`` was non-nanosecond (:issue:`59036`)
 - Bug in :meth:`Series.dt.microsecond` producing incorrect results for pyarrow backed :class:`Series`. (:issue:`59154`)
+- Bug in :meth:`_validate_inferred_freq` where DateTimeIndex incorrectly raised a ``ValueError`` when assigning a logically equivalent frequency (:issue:`61086`)
 - Bug in :meth:`to_datetime` not respecting dayfirst if an uncommon date string was passed. (:issue:`58859`)
 - Bug in :meth:`to_datetime` on float array with missing values throwing ``FloatingPointError`` (:issue:`58419`)
 - Bug in :meth:`to_datetime` on float32 df with year, month, day etc. columns leads to precision issues and incorrect result. (:issue:`60506`)

pandas/core/arrays/datetimelike.py

@@ -2519,8 +2519,13 @@ def _validate_inferred_freq(
     -------
     freq : DateOffset or None
     """
+    offset1 = to_offset(freq)
+    offset2 = to_offset(inferred_freq)
+
+    freq_equal = type(offset1) == type(offset2)
+
     if inferred_freq is not None:
-        if freq is not None and freq != inferred_freq:
+        if freq is not None and not freq_equal:
             raise ValueError(
                 f"Inferred frequency {inferred_freq} from passed "
                 "values does not conform to passed frequency "

pandas/tests/indexes/datetimes/test_constructors.py

@@ -1204,3 +1204,10 @@ def test_dti_constructor_object_dtype_dayfirst_yearfirst_with_tz(self):
         result2 = DatetimeIndex([val], tz="US/Pacific", yearfirst=True)
         expected2 = DatetimeIndex([yfirst]).as_unit("s")
         tm.assert_index_equal(result2, expected2)
+
+    def test_validate_inferred_freq_equivalence(self):
+        idx = date_range("2020-02-01", freq="QS-FEB", periods=4)
+
+        new_idx = DatetimeIndex(idx, freq="QS-MAY")
+
+        assert isinstance(new_idx, DatetimeIndex)