BUG: fix crash when using arange on datetime without dtype set

numpy/core/src/multiarray/datetime.c

@@ -3047,7 +3047,7 @@ cast_timedelta_to_timedelta(PyArray_DatetimeMetaData *src_meta,
  * Returns true if the object is something that is best considered
  * a Datetime, false otherwise.
  */
-static npy_bool
+static NPY_GCC_NONNULL(1) npy_bool
 is_any_numpy_datetime(PyObject *obj)
 {
     return (PyArray_IsScalar(obj, Datetime) ||
@@ -3296,7 +3296,8 @@ datetime_arange(PyObject *start, PyObject *stop, PyObject *step,
         }
     }
     else {
-        if (is_any_numpy_datetime(start) || is_any_numpy_datetime(stop)) {
+        if ((start && is_any_numpy_datetime(start)) ||
+            is_any_numpy_datetime(stop)) {
             type_nums[0] = NPY_DATETIME;
         }
         else {

numpy/core/tests/test_datetime.py

@@ -1412,6 +1412,11 @@ def test_datetime_arange(self):
                                 np.datetime64('2012-02-03T14Z', 's'),
                                 np.timedelta64(5, 'Y'))
 
+    def test_datetime_arange_no_dtype(self):
+        d = np.array('2010-01-04', dtype="M8[D]")
+        assert_equal(np.arange(d, d + 1), d)
+        assert_raises(ValueError, np.arange, d)
+
     def test_timedelta_arange(self):
         a = np.arange(3, 10, dtype='m8')
         assert_equal(a.dtype, np.dtype('m8'))
@@ -1430,6 +1435,11 @@ def test_timedelta_arange(self):
         assert_raises(TypeError, np.arange, np.timedelta64(0, 'Y'),
                                 np.timedelta64(5, 'D'))
 
+    def test_timedelta_arange_no_dtype(self):
+        d = np.array(5, dtype="m8[D]")
+        assert_equal(np.arange(d, d + 1), d)
+        assert_raises(ValueError, np.arange, d)
+
     def test_datetime_maximum_reduce(self):
         a = np.array(['2010-01-02', '1999-03-14', '1833-03'], dtype='M8[D]')
         assert_equal(np.maximum.reduce(a).dtype, np.dtype('M8[D]'))