add commented fix if merging after gh-124652

Author: dg-pb

Modules/_functoolsmodule.c

@@ -391,9 +391,19 @@ partial_vectorcall(partialobject *pto, PyObject *const *args,
                                               pto_nargs, NULL);
         }
 
+        // TO BE ADDED ON MERGE TO MAIN
+        //      IF https://github.com/python/cpython/pull/124788
+        //      IS MERGED BEFORE THIS
+        // /* Fast path if all Placeholders */
+        // if (pto_nargs == pto_phcount) {
+        //     /* NOTE: Without this, following single argument Fast Path
+        //      * is incorrect */
+        //     return _PyObject_VectorcallTstate(tstate, pto->fn,
+        //                                       args, nargs, kwnames);
+        // }
+
         /* Use PY_VECTORCALL_ARGUMENTS_OFFSET to prepend a single
-         * positional argument.
-         * NOTE: Could add (Placeholder, arg) case in similar manner */
+         * positional argument. */
         if (pto_nargs == 1 && (nargsf & PY_VECTORCALL_ARGUMENTS_OFFSET)) {
             PyObject **newargs = (PyObject **)args - 1;
             PyObject *tmp = newargs[0];