Fix pandas-dev#17965 datetime64 comparison

Author: watercrossing

doc/source/whatsnew/v0.21.1.txt

@@ -61,6 +61,7 @@ Bug Fixes
 - Bug in :class:`DatetimeIndex` subtracting datetimelike from DatetimeIndex could fail to overflow (:issue:`18020`)
 - Bug in ``pd.Series.rolling.skew()`` and ``rolling.kurt()`` with all equal values has floating issue (:issue:`18044`)
 - Bug in ``pd.DataFrameGroupBy.count()`` when counting over a datetimelike column (:issue:`13393`)
+- Bug in ``_libs.index.convert_scalar()`` when comparing datetimes to np.datetime64 (:issue:`17965`)
 
 Conversion
 ^^^^^^^^^^

pandas/_libs/index.pyx

@@ -549,6 +549,8 @@ cpdef convert_scalar(ndarray arr, object value):
     if arr.descr.type_num == NPY_DATETIME:
         if isinstance(value, np.ndarray):
             pass
+        if isinstance(value, np.datetime64):
+            return Timestamp(value).value
         elif isinstance(value, datetime):
             return Timestamp(value).value
         elif value is None or value != value:

pandas/tests/indexes/datetimes/test_partial_slicing.py

@@ -330,3 +330,9 @@ def test_loc_datetime_length_one(self):
 
         result = df.loc['2016-10-01T00:00:00':]
         tm.assert_frame_equal(result, df)
+
+    def test_comparison_to_datetime64(self):
+        # GH issue #17965
+        df = DataFrame({'dates': date_range('2000-01-01', '2000-01-05')})
+        date = df['dates'].unique()[0]
+        tm.assert_frame_equal(df.loc[df['dates'] == date], df.iloc[:1])