gh-158486: Fix PowerSort mask calculation for very large lists

Author: neelsomani

Misc/NEWS.d/next/Core_and_Builtins/2026-09-30-01-30-58.gh-issue-158486.cqmMwv.rst

@@ -0,0 +1,2 @@
+Fix the PowerSort mask calculation in :meth:`list.sort` for very large
+lists.

Objects/listobject.c

@@ -2263,7 +2263,7 @@ merge_init(MergeState *ms, Py_ssize_t list_size, int has_keyfunc,
     while (list_size >> ms->mr_e >= MAX_MINRUN) {
         ++ms->mr_e;
     }
-    ms->mr_mask = (1 << ms->mr_e) - 1;
+    ms->mr_mask = ((Py_ssize_t)1 << ms->mr_e) - 1;
     ms->mr_current = 0;
 }