Issue #27214: Fix potential bug and remove useless optimization in long_invert. Thanks Oren Milman.

Misc/NEWS

@@ -10,6 +10,10 @@ What's New in Python 3.6.0 beta 1
 Core and Builtins
 -----------------
 
+- Issue #27214: In long_invert, be more careful about modifying object
+  returned by long_add, and remove an unnecessary check for small longs.
+  Thanks Oren Milman for analysis and patch.
+
 - Issue #27506: Support passing the bytes/bytearray.translate() "delete"
   argument by keyword.
 

Objects/longobject.c

@@ -4170,8 +4170,10 @@ long_invert(PyLongObject *v)
     Py_DECREF(w);
     if (x == NULL)
         return NULL;
-    Py_SIZE(x) = -(Py_SIZE(x));
-    return (PyObject *)maybe_small_long(x);
+    _PyLong_Negate(&x);
+    /* No need for maybe_small_long here, since any small
+       longs will have been caught in the Py_SIZE <= 1 fast path. */
+    return (PyObject *)x;
 }
 
 static PyObject *