Fix a SystemError in code.replace() (#27771)

While the comment said 'We don't bother resizing localspluskinds',
this would cause .replace() to crash when it happened.
(Also types.CodeType(), but testing that is tedious, and this tests all
code paths.)

2 files changed, 14 insertions, 3 deletions

diff --git a/Lib/test/test_code.py b/Lib/test/test_code.py
index 2bdd7e7494..9d6b465115 100644
--- a/Lib/test/test_code.py
+++ b/Lib/test/test_code.py
@@ -320,6 +320,15 @@ class CodeTest(unittest.TestCase):
         with self.assertRaises(ValueError):
             co.replace(co_nlocals=co.co_nlocals + 1)
 
+    def test_shrinking_localsplus(self):
+        # Check that PyCode_NewWithPosOnlyArgs resizes both
+        # localsplusnames and localspluskinds, if an argument is a cell.
+        def func(arg):
+            return lambda: arg
+        code = func.__code__
+        newcode = code.replace(co_name="func")  # Should not raise SystemError
+        self.assertEqual(code, newcode)
+
     def test_empty_linetable(self):
         def func():
             pass
diff --git a/Objects/codeobject.c b/Objects/codeobject.c
index 3dc9fd787f..ad8f13a781 100644
--- a/Objects/codeobject.c
+++ b/Objects/codeobject.c
@@ -471,9 +471,11 @@ PyCode_NewWithPosOnlyArgs(int argcount, int posonlyargcount, int kwonlyargcount,
                                localsplusnames, localspluskinds);
     }
     // If any cells were args then nlocalsplus will have shrunk.
-    // We don't bother resizing localspluskinds.
-    if (_PyTuple_Resize(&localsplusnames, nlocalsplus) < 0) {
-        goto error;
+    if (nlocalsplus != PyTuple_GET_SIZE(localsplusnames)) {
+        if (_PyTuple_Resize(&localsplusnames, nlocalsplus) < 0
+                || _PyBytes_Resize(&localspluskinds, nlocalsplus) < 0) {
+            goto error;
+        }
     }
 
     struct _PyCodeConstructor con = {