Pandas Regression: cannot set a Timestamp with a non-timestamp

Code Sample, a copy-pastable example if possible

import pandas as pd
from datetime import datetime

dates = [pd.Timestamp('2012-01-01'), pd.Timestamp('2017-01-02'), pd.Timestamp('2017-01-03')]
df = pd.DataFrame({'dates': dates})
today = datetime.today().date()
print(df['dates'] == today)

Traceback (most recent call last):
  File "date_bug.py", line 22, in <module>
    main()
  File "date_bug.py", line 18, in main
    print(df['dates'] == today)
  File "/Users/discort/python/projects/pims/env/lib/python3.5/site-packages/pandas/core/ops.py", line 879, in wrapper
    res = na_op(values, other)
  File "/Users/discort/python/projects/pims/env/lib/python3.5/site-packages/pandas/core/ops.py", line 808, in na_op
    y = libindex.convert_scalar(x, _values_from_object(y))
  File "pandas/_libs/index.pyx", line 494, in pandas._libs.index.convert_scalar
  File "pandas/_libs/index.pyx", line 509, in pandas._libs.index.convert_scalar
ValueError: cannot set a Timestamp with a non-timestamp

Problem description

There is a regression. Related: #17965 On 0.18.1 it works fine

Expected Output

0    False
1    False
2    False
Name: dates, dtype: bool

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None pandas: 0.21.0 pytest: 2.9.2 pip: 9.0.1 setuptools: 36.4.0 Cython: None numpy: 1.13.3 scipy: None pyarrow: None xarray: None IPython: 6.1.0 sphinx: None patsy: None dateutil: 2.6.1 pytz: 2017.3 blosc: None bottleneck: None tables: None numexpr: None feather: None matplotlib: None openpyxl: None xlrd: None xlwt: None xlsxwriter: None lxml: None bs4: None html5lib: None sqlalchemy: None pymysql: None psycopg2: None jinja2: None s3fs: None fastparquet: None pandas_gbq: None pandas_datareader: None

Comment From: jreback

as indicated this is a dupe of #17965, closed by #18188 and will be in forthcoming 0.21.1