ConfigParser does not handle square brackets in section name

[xflr6]

BPO	23301
Nosy	@bitdancer, @xflr6
Superseder	bpo-20923: [doc] Explain ConfigParser 'valid section name' and .SECTCRE

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2015-01-22.16:06:52.188>
created_at = <Date 2015-01-22.15:58:05.320>
labels = ['type-bug', 'library']
title = 'ConfigParser does not handle square brackets in section name'
updated_at = <Date 2015-01-22.16:06:52.186>
user = 'https://github.com/xflr6'

bugs.python.org fields:

activity = <Date 2015-01-22.16:06:52.186>
actor = 'r.david.murray'
assignee = 'none'
closed = True
closed_date = <Date 2015-01-22.16:06:52.188>
closer = 'r.david.murray'
components = ['Library (Lib)']
creation = <Date 2015-01-22.15:58:05.320>
creator = 'xflr6'
dependencies = []
files = []
hgrepos = []
issue_num = 23301
keywords = []
message_count = 2.0
messages = ['234497', '234498']
nosy_count = 2.0
nosy_names = ['r.david.murray', 'xflr6']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '20923'
type = 'behavior'
url = 'https://bugs.python.org/issue23301'
versions = ['Python 2.7', 'Python 3.4']

[xflr6]

ConfigParser parses section lines containing square brackets like '[spam [eggs] spam]' up to the first instead of the last occurrence of ']' preventing roundtrips:

>> s = StringIO()
>> c1 = ConfigParser()
>> c1.add_section('spam [eggs]')
>> c1.write(s)
>> s.seek(0)
>> c2 = ConfigParser()
>> c2.readfp(s)
>> assert c1.sections() == c2.sections() # fails

Potential fix: change the second line of SECTCRE from r'(?P<header>[^]]+)' to r'(?P<header>.+?)'.

If the parsing behaviour cannot be changed, the user should at least be warned about supplying data that breaks the roundtrip.

[bitdancer]

This is a duplicate of bpo-20923 (#65122), which was rejected. To argue against the rejection you probably need to provide evidence that this is something that is actually supported by other common ini parsers. And that evidence should be posted to bpo-20923.