bug-DatetimeIndex-is_year_start-breaks-on-freq-BusinessMonthStart

Author: natmokval

pandas/_libs/tslibs/fields.pyx

@@ -253,7 +253,8 @@ def get_start_end_field(
         # month of year. Other offsets use month, startingMonth as ending
         # month of year.
 
-        if freq_name.lstrip("B")[0:2] in ["MS", "QS", "YS"]:
+        if (freq_name.lstrip("B")[0:2] in ["MS", "QS", "YS"]) or (
+                freq_name.lstrip("CB")[0:2] in ["MS", "QS", "YS"]):
             end_month = 12 if month_kw == 1 else month_kw - 1
             start_month = month_kw
         else:

pandas/tests/indexes/datetimes/test_scalar_compat.py

@@ -343,3 +343,8 @@ def test_dti_is_year_start_freq_custom_business_day_with_digit(self):
         msg = "Custom business days is not supported by is_year_start"
         with pytest.raises(ValueError, match=msg):
             dr.is_year_start
+
+    def test_dti_is_year_start_freq_business_month_start_with(self):
+        dr = date_range("2020-01-01", periods=1, freq="CBMS")
+        print(dr.is_year_start)
+        # assert all(dr.is_year_start)