assert that _simple_new always receives an array of datetime64[ns]

Author: reidy-p

pandas/core/indexes/datetimes.py

@@ -559,6 +559,7 @@ def _generate(cls, start, end, periods, name, freq,
                     arr = arr.view(_NS_DTYPE)
                     index = DatetimeIndex(arr)
 
+
                     # index is localized datetime64 array -> have to convert
                     # start/end as well to compare
                     if start is not None:
@@ -611,8 +612,8 @@ def _simple_new(cls, values, name=None, freq=None, tz=None,
                            dtype=dtype, **kwargs)
             values = np.array(values, copy=False)
 
-        # values should be a numpy array
-        assert isinstance(values, np.ndarray)
+        assert isinstance(values, np.ndarray), "values is not an np.ndarray"
+        assert is_datetime64_dtype(values)
 
         if is_object_dtype(values):
             return cls(values, name=name, freq=freq, tz=tz,
@@ -2094,6 +2095,8 @@ def _generate_regular_range(start, end, periods, freq):
                              "if a 'period' is given.")
 
         data = np.arange(b, e, stride, dtype=np.int64)
+
+        # _simple_new is getting an array of int64 here
         data = DatetimeIndex._simple_new(data, None, tz=tz)
     else:
         if isinstance(start, Timestamp):