diff -r 0db4403e62c4 Parser/asdl_c.py
--- a/Parser/asdl_c.py	Tue Sep 27 05:10:40 2016 +0000
+++ b/Parser/asdl_c.py	Tue Sep 27 11:02:28 2016 +0300
@@ -526,6 +526,13 @@ class Obj2ModVisitor(PickleVisitor):
             self.emit("res = obj2ast_%s(PyList_GET_ITEM(tmp, i), &value, arena);" %
                       field.type, depth+2, reflow=False)
             self.emit("if (res != 0) goto failed;", depth+2)
+            self.emit("if (len != PyList_GET_SIZE(tmp)) {", depth+2)
+            self.emit("PyErr_SetString(PyExc_RuntimeError, \"%s field \\\"%s\\\" "
+                      "changed size during iteration\");" %
+                      (name, field.name),
+                      depth+3, reflow=False)
+            self.emit("goto failed;", depth+3)
+            self.emit("}", depth+2)
             self.emit("asdl_seq_SET(%s, i, value);" % field.name, depth+2)
             self.emit("}", depth+1)
         else:
diff -r 0db4403e62c4 Python/Python-ast.c
--- a/Python/Python-ast.c	Tue Sep 27 05:10:40 2016 +0000
+++ b/Python/Python-ast.c	Tue Sep 27 11:02:28 2016 +0300
@@ -3797,6 +3797,10 @@ obj2ast_mod(PyObject* obj, mod_ty* out, 
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Module field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -3832,6 +3836,10 @@ obj2ast_mod(PyObject* obj, mod_ty* out, 
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Interactive field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -3889,6 +3897,10 @@ obj2ast_mod(PyObject* obj, mod_ty* out, 
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Suite field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -3992,6 +4004,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "FunctionDef field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -4016,6 +4032,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "FunctionDef field \"decorator_list\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(decorator_list, i, value);
             }
             Py_CLEAR(tmp);
@@ -4088,6 +4108,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "AsyncFunctionDef field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -4112,6 +4136,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "AsyncFunctionDef field \"decorator_list\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(decorator_list, i, value);
             }
             Py_CLEAR(tmp);
@@ -4173,6 +4201,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "ClassDef field \"bases\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(bases, i, value);
             }
             Py_CLEAR(tmp);
@@ -4197,6 +4229,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 keyword_ty value;
                 res = obj2ast_keyword(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "ClassDef field \"keywords\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(keywords, i, value);
             }
             Py_CLEAR(tmp);
@@ -4221,6 +4257,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "ClassDef field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -4245,6 +4285,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "ClassDef field \"decorator_list\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(decorator_list, i, value);
             }
             Py_CLEAR(tmp);
@@ -4302,6 +4346,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Delete field \"targets\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(targets, i, value);
             }
             Py_CLEAR(tmp);
@@ -4338,6 +4386,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Assign field \"targets\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(targets, i, value);
             }
             Py_CLEAR(tmp);
@@ -4455,6 +4507,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "For field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -4479,6 +4535,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "For field \"orelse\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(orelse, i, value);
             }
             Py_CLEAR(tmp);
@@ -4539,6 +4599,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "AsyncFor field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -4563,6 +4627,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "AsyncFor field \"orelse\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(orelse, i, value);
             }
             Py_CLEAR(tmp);
@@ -4611,6 +4679,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "While field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -4635,6 +4707,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "While field \"orelse\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(orelse, i, value);
             }
             Py_CLEAR(tmp);
@@ -4683,6 +4759,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "If field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -4707,6 +4787,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "If field \"orelse\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(orelse, i, value);
             }
             Py_CLEAR(tmp);
@@ -4743,6 +4827,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 withitem_ty value;
                 res = obj2ast_withitem(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "With field \"items\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(items, i, value);
             }
             Py_CLEAR(tmp);
@@ -4767,6 +4855,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "With field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -4803,6 +4895,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 withitem_ty value;
                 res = obj2ast_withitem(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "AsyncWith field \"items\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(items, i, value);
             }
             Py_CLEAR(tmp);
@@ -4827,6 +4923,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "AsyncWith field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -4897,6 +4997,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Try field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -4921,6 +5025,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 excepthandler_ty value;
                 res = obj2ast_excepthandler(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Try field \"handlers\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(handlers, i, value);
             }
             Py_CLEAR(tmp);
@@ -4945,6 +5053,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Try field \"orelse\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(orelse, i, value);
             }
             Py_CLEAR(tmp);
@@ -4969,6 +5081,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Try field \"finalbody\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(finalbody, i, value);
             }
             Py_CLEAR(tmp);
@@ -5038,6 +5154,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 alias_ty value;
                 res = obj2ast_alias(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Import field \"names\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(names, i, value);
             }
             Py_CLEAR(tmp);
@@ -5085,6 +5205,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 alias_ty value;
                 res = obj2ast_alias(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "ImportFrom field \"names\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(names, i, value);
             }
             Py_CLEAR(tmp);
@@ -5130,6 +5254,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 identifier value;
                 res = obj2ast_identifier(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Global field \"names\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(names, i, value);
             }
             Py_CLEAR(tmp);
@@ -5165,6 +5293,10 @@ obj2ast_stmt(PyObject* obj, stmt_ty* out
                 identifier value;
                 res = obj2ast_identifier(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Nonlocal field \"names\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(names, i, value);
             }
             Py_CLEAR(tmp);
@@ -5306,6 +5438,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "BoolOp field \"values\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(values, i, value);
             }
             Py_CLEAR(tmp);
@@ -5502,6 +5638,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Dict field \"keys\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(keys, i, value);
             }
             Py_CLEAR(tmp);
@@ -5526,6 +5666,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Dict field \"values\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(values, i, value);
             }
             Py_CLEAR(tmp);
@@ -5561,6 +5705,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Set field \"elts\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(elts, i, value);
             }
             Py_CLEAR(tmp);
@@ -5608,6 +5756,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 comprehension_ty value;
                 res = obj2ast_comprehension(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "ListComp field \"generators\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(generators, i, value);
             }
             Py_CLEAR(tmp);
@@ -5655,6 +5807,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 comprehension_ty value;
                 res = obj2ast_comprehension(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "SetComp field \"generators\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(generators, i, value);
             }
             Py_CLEAR(tmp);
@@ -5714,6 +5870,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 comprehension_ty value;
                 res = obj2ast_comprehension(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "DictComp field \"generators\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(generators, i, value);
             }
             Py_CLEAR(tmp);
@@ -5761,6 +5921,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 comprehension_ty value;
                 res = obj2ast_comprehension(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "GeneratorExp field \"generators\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(generators, i, value);
             }
             Py_CLEAR(tmp);
@@ -5874,6 +6038,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 cmpop_ty value;
                 res = obj2ast_cmpop(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Compare field \"ops\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(ops, i, value);
             }
             Py_CLEAR(tmp);
@@ -5898,6 +6066,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Compare field \"comparators\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(comparators, i, value);
             }
             Py_CLEAR(tmp);
@@ -5946,6 +6118,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Call field \"args\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(args, i, value);
             }
             Py_CLEAR(tmp);
@@ -5970,6 +6146,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 keyword_ty value;
                 res = obj2ast_keyword(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Call field \"keywords\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(keywords, i, value);
             }
             Py_CLEAR(tmp);
@@ -6264,6 +6444,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "List field \"elts\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(elts, i, value);
             }
             Py_CLEAR(tmp);
@@ -6311,6 +6495,10 @@ obj2ast_expr(PyObject* obj, expr_ty* out
                 expr_ty value;
                 res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "Tuple field \"elts\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(elts, i, value);
             }
             Py_CLEAR(tmp);
@@ -6476,6 +6664,10 @@ obj2ast_slice(PyObject* obj, slice_ty* o
                 slice_ty value;
                 res = obj2ast_slice(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "ExtSlice field \"dims\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(dims, i, value);
             }
             Py_CLEAR(tmp);
@@ -6835,6 +7027,10 @@ obj2ast_comprehension(PyObject* obj, com
             expr_ty value;
             res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
             if (res != 0) goto failed;
+            if (len != PyList_GET_SIZE(tmp)) {
+                PyErr_SetString(PyExc_RuntimeError, "comprehension field \"ifs\" changed size during iteration");
+                goto failed;
+            }
             asdl_seq_SET(ifs, i, value);
         }
         Py_CLEAR(tmp);
@@ -6930,6 +7126,10 @@ obj2ast_excepthandler(PyObject* obj, exc
                 stmt_ty value;
                 res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                 if (res != 0) goto failed;
+                if (len != PyList_GET_SIZE(tmp)) {
+                    PyErr_SetString(PyExc_RuntimeError, "ExceptHandler field \"body\" changed size during iteration");
+                    goto failed;
+                }
                 asdl_seq_SET(body, i, value);
             }
             Py_CLEAR(tmp);
@@ -6976,6 +7176,10 @@ obj2ast_arguments(PyObject* obj, argumen
             arg_ty value;
             res = obj2ast_arg(PyList_GET_ITEM(tmp, i), &value, arena);
             if (res != 0) goto failed;
+            if (len != PyList_GET_SIZE(tmp)) {
+                PyErr_SetString(PyExc_RuntimeError, "arguments field \"args\" changed size during iteration");
+                goto failed;
+            }
             asdl_seq_SET(args, i, value);
         }
         Py_CLEAR(tmp);
@@ -7010,6 +7214,10 @@ obj2ast_arguments(PyObject* obj, argumen
             arg_ty value;
             res = obj2ast_arg(PyList_GET_ITEM(tmp, i), &value, arena);
             if (res != 0) goto failed;
+            if (len != PyList_GET_SIZE(tmp)) {
+                PyErr_SetString(PyExc_RuntimeError, "arguments field \"kwonlyargs\" changed size during iteration");
+                goto failed;
+            }
             asdl_seq_SET(kwonlyargs, i, value);
         }
         Py_CLEAR(tmp);
@@ -7034,6 +7242,10 @@ obj2ast_arguments(PyObject* obj, argumen
             expr_ty value;
             res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
             if (res != 0) goto failed;
+            if (len != PyList_GET_SIZE(tmp)) {
+                PyErr_SetString(PyExc_RuntimeError, "arguments field \"kw_defaults\" changed size during iteration");
+                goto failed;
+            }
             asdl_seq_SET(kw_defaults, i, value);
         }
         Py_CLEAR(tmp);
@@ -7068,6 +7280,10 @@ obj2ast_arguments(PyObject* obj, argumen
             expr_ty value;
             res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
             if (res != 0) goto failed;
+            if (len != PyList_GET_SIZE(tmp)) {
+                PyErr_SetString(PyExc_RuntimeError, "arguments field \"defaults\" changed size during iteration");
+                goto failed;
+            }
             asdl_seq_SET(defaults, i, value);
         }
         Py_CLEAR(tmp);