DatetimeIndex + DateOffset broken around DST switching time

[torfsen]

Code Sample, a copy-pastable example if possible

>>> import pandas as pd
>>> idx = pd.DatetimeIndex(['2000-03-26 04:00'], tz='Europe/Berlin')  # Switch to DST was on that day
>>> idx
DatetimeIndex(['2000-03-26 04:00:00+02:00'], dtype='datetime64[ns, Europe/Berlin]', freq=None)
>>> idx + pd.DateOffset(hours=-2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/var/local/conda/envs/pandas-debug/lib/python3.7/site-packages/pandas/core/indexes/datetimelike.py", line 510, in __add__
    result = self._data.__add__(maybe_unwrap_index(other))
  File "/var/local/conda/envs/pandas-debug/lib/python3.7/site-packages/pandas/core/arrays/datetimelike.py", line 1212, in __add__
    result = self._add_offset(other)
  File "/var/local/conda/envs/pandas-debug/lib/python3.7/site-packages/pandas/core/arrays/datetimes.py", line 832, in _add_offset
    result = result.tz_localize(self.tz)
  File "/var/local/conda/envs/pandas-debug/lib/python3.7/site-packages/pandas/core/arrays/datetimes.py", line 1151, in tz_localize
    self.asi8, tz, ambiguous=ambiguous, nonexistent=nonexistent
  File "pandas/_libs/tslibs/tzconversion.pyx", line 276, in pandas._libs.tslibs.tzconversion.tz_localize_to_utc
pytz.exceptions.NonExistentTimeError: 2000-03-26 02:00:00
>>> idx[0] + pd.DateOffset(hours=-2)
Timestamp('2000-03-26 01:00:00+0100', tz='Europe/Berlin')

Problem description

The vectorized operation DatetimeIndex + DateOffset is broken around the DST switching time. However, the same operation works element-wise (Timestamp + DateOffset), so I would expect either both versions to work (preferably) or both versions to fail with the same error.

Expected Output

DatetimeIndex(['2000-03-26 01:00:00+01:00'], dtype='datetime64[ns, Europe/Berlin]', freq=None)

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.7.4.final.0
python-bits : 64
OS : Linux
OS-release : 5.0.0-29-generic
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 0.25.1
numpy : 1.16.5
pytz : 2019.2
dateutil : 2.8.0
pip : 19.2.3
setuptools : 41.2.0
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None

[mroeschke]

The issue is in this line:

pandas/pandas/core/arrays/datetimes.py

Line 830 in 83eb75b

result = result.tz_localize(self.tz)

tz_localize has keyword argument ambiguous and nonexistent arguments to handle these cases.