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

Doc/c-api/slice.rst

@@ -75,7 +75,7 @@ Slice Objects
    Extract the start, stop and step data members from a slice object as
    C integers.  Silently reduce values larger than ``PY_SSIZE_T_MAX`` to
    ``PY_SSIZE_T_MAX``, silently boost the start and stop values less than
-   ``-PY_SSIZE_T_MAX-1`` to ``-PY_SSIZE_T_MAX-1``, and silently boost the step
+   ``PY_SSIZE_T_MIN`` to ``PY_SSIZE_T_MIN``, and silently boost the step
    values less than ``-PY_SSIZE_T_MAX`` to ``-PY_SSIZE_T_MAX``.
 
    Return ``-1`` on error, ``0`` on success.

Objects/sliceobject.c

@@ -197,6 +197,8 @@ PySlice_Unpack(PyObject *_r,
     PySliceObject *r = (PySliceObject*)_r;
     /* this is harder to get right than you might think */
 
+    Py_BUILD_ASSERT(PY_SSIZE_T_MIN + 1 <= -PY_SSIZE_T_MAX);
+
     if (r->step == Py_None) {
         *step = 1;
     }
@@ -217,14 +219,14 @@ 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;

Python/ceval.c

@@ -4889,7 +4889,7 @@ do_call_core(PyObject *func, PyObject *callargs, PyObject *kwdict)
 /* Extract a slice index from a 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.
 */
 int