Undefined C behavior going beyond end of struct via a [1] arrays (C99 flexible arrays)

GitHub fields:

assignee = 'https://github.com/gpshead'
closed_at = None
created_at = <Date 2020-03-30.21:04:23.697>
labels = ['interpreter-core', 'build', '3.11']
title = 'Undefined C behavior going beyond end of struct via a [1] arrays.'
updated_at = <Date 2022-01-21.23:32:19.727>
user = 'https://github.com/gpshead'

bugs.python.org fields:

activity = <Date 2022-01-21.23:32:19.727>
actor = 'gregory.p.smith'
assignee = 'gregory.p.smith'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2020-03-30.21:04:23.697>
creator = 'gregory.p.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 40120
keywords = ['patch']
message_count = 20.0
messages = ['365349', '365350', '365399', '365403', '365410', '365415', '365425', '365464', '365465', '365466', '365472', '365539', '365671', '365691', '365744', '365745', '366883', '411151', '411168', '411207']
nosy_count = 6.0
nosy_names = ['gregory.p.smith', 'pitrou', 'vstinner', 'serhiy.storchaka', 'colesbury', 'pablogsal']
pr_nums = ['19232']
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue40120'
versions = ['Python 3.11']