BUG: fix tz-aware DatetimeIndex + TimedeltaIndex (#17558)

Fix minor bug causing DatetimeIndex + TimedeltaIndex to raise an error,
and fix another bug causing the sum of a tz-aware DatetimeIndex and a
numpy array of timedeltas to incorrectly have timezone applied twice.

Author: azjps

doc/source/whatsnew/v0.21.0.txt

@@ -506,6 +506,7 @@ Indexing
 - Bug in ``CategoricalIndex`` reindexing in which specified indices containing duplicates were not being respected (:issue:`17323`)
 - Bug in intersection of ``RangeIndex`` with negative step (:issue:`17296`)
 - Bug in ``IntervalIndex`` where performing a scalar lookup fails for included right endpoints of non-overlapping monotonic decreasing indexes (:issue:`16417`, :issue:`17271`)
+- Bug in adding ``DatetimeIndex`` with a ``TimedeltaIndex`` or a numpy array with ``dtype="timedelta64"`` (:issue:`17558`)
 
 I/O
 ^^^

pandas/core/indexes/datetimelike.py

@@ -13,6 +13,7 @@
     is_integer, is_float,
     is_bool_dtype, _ensure_int64,
     is_scalar, is_dtype_equal,
+    is_timedelta64_dtype, is_integer_dtype,
     is_list_like)
 from pandas.core.dtypes.generic import (
     ABCIndex, ABCSeries,
@@ -651,6 +652,15 @@ def __add__(self, other):
                 raise TypeError("cannot add {typ1} and {typ2}"
                                 .format(typ1=type(self).__name__,
                                         typ2=type(other).__name__))
+            elif isinstance(other, np.ndarray):
+                if is_timedelta64_dtype(other):
+                    return self._add_delta(TimedeltaIndex(other))
+                elif is_integer_dtype(other):
+                    return NotImplemented
+                else:
+                    raise TypeError("cannot add {typ1} and np.ndarray[{typ2}]"
+                                    .format(typ1=type(self).__name__,
+                                            typ2=other.dtype))
             elif isinstance(other, (DateOffset, timedelta, np.timedelta64,
                                     Timedelta)):
                 return self._add_delta(other)
@@ -731,7 +741,7 @@ def _add_delta_tdi(self, other):
         if self.hasnans or other.hasnans:
             mask = (self._isnan) | (other._isnan)
             new_values[mask] = iNaT
-        return new_values.view(self.dtype)
+        return new_values.view('i8')
 
     def isin(self, values):
         """

pandas/tests/indexes/datetimes/test_ops.py

@@ -460,6 +460,20 @@ def test_add_dti_dti(self):
         with pytest.raises(TypeError):
             dti + dti_tz
 
+    def test_add_dti_ndarray(self):
+        # GH 17558
+        # Check that tz-aware DatetimeIndex + np.array(dtype="timedelta64")
+        # and DatetimeIndex + TimedeltaIndex work as expected
+        dti = pd.DatetimeIndex([pd.Timestamp("2017/01/01")])
+        dti = dti.tz_localize('US/Eastern')
+        expected = pd.DatetimeIndex([pd.Timestamp("2017/01/01 01:00")])
+        expected = expected.tz_localize('US/Eastern')
+
+        td_np = np.array([np.timedelta64(1, 'h')], dtype="timedelta64[ns]")
+        tm.assert_index_equal(dti + td_np, expected)
+        tm.assert_index_equal(dti + td_np[0], expected)
+        tm.assert_index_equal(dti + TimedeltaIndex(td_np), expected)
+
     def test_difference(self):
         for tz in self.tz:
             # diff