TST: add tests for DatetimeIndex.is_year_start/is_quarter_start on "BMS" frequency

pandas/tests/indexes/datetimes/test_scalar_compat.py

@@ -424,6 +424,17 @@ def test_dti_is_year_start_freq_custom_business_day_with_digit(self):
  with pytest.raises(ValueError, match=msg):
  dr.is_year_start
 
+ @pytest.mark.parametrize("freq", ["3BMS", offsets.BusinessMonthBegin(3)])
+ def test_dti_is_year_quarter_start_freq_business_month_begin(self, freq):
+ # GH#58729
+ dr = date_range("2020-01-01", periods=5, freq=freq)
+ result = [x.is_year_start for x in dr]
+ assert result == [True, False, False, False, True]
+
+ dr = date_range("2020-01-01", periods=4, freq=freq)
+ result = [x.is_quarter_start for x in dr]
+ assert all(dr.is_quarter_start)
+
 
 @given(
  dt=st.datetimes(min_value=datetime(1960, 1, 1), max_value=datetime(1980, 1, 1)),