bpo-30281: Fix the default value for stop in PySlice_Unpack() (#1531) (#1480)

Objects/sliceobject.c

@@ -137,6 +137,8 @@ _PySlice_Unpack(PyObject *_r,
     PySliceObject *r = (PySliceObject *)_r;
     /* this is harder to get right than you might think */
 
+    assert(PY_SSIZE_T_MIN + 1 <= -PY_SSIZE_T_MAX);
+
     if (r->step == Py_None) {
         *step = 1;
     }
@@ -157,14 +159,14 @@ _PySlice_Unpack(PyObject *_r,
     }
 
     if (r->start == Py_None) {
-        *start = *step < 0 ? PY_SSIZE_T_MAX-1 : 0;;
+        *start = *step < 0 ? PY_SSIZE_T_MAX : 0;
     }
     else {
         if (!_PyEval_SliceIndex(r->start, start)) return -1;
     }
 
     if (r->stop == Py_None) {
-        *stop = *step < 0 ? -PY_SSIZE_T_MAX : PY_SSIZE_T_MAX;
+        *stop = *step < 0 ? PY_SSIZE_T_MIN : PY_SSIZE_T_MAX;
     }
     else {
         if (!_PyEval_SliceIndex(r->stop, stop)) return -1;
@@ -198,7 +200,7 @@ _PySlice_AdjustIndices(Py_ssize_t length,
             *stop = (step < 0) ? -1 : 0;
         }
     }
-    else  if (*stop >= length) {
+    else if (*stop >= length) {
         *stop = (step < 0) ? length - 1 : length;
     }
 

Python/ceval.c

@@ -4679,7 +4679,7 @@ ext_call_fail:
 /* Extract a slice index from a PyInt or PyLong or an object with the
    nb_index slot defined, and store in *pi.
    Silently reduce values larger than PY_SSIZE_T_MAX to PY_SSIZE_T_MAX,
-   and silently boost values less than -PY_SSIZE_T_MAX-1 to -PY_SSIZE_T_MAX-1.
+   and silently boost values less than PY_SSIZE_T_MIN to PY_SSIZE_T_MIN.
    Return 0 on error, 1 on success.
 */
 /* Note:  If v is NULL, return success without storing into *pi.  This