enum: Adding a member named _classname__ raises IndexError

[smurfix]

BPO	43744
Nosy	@warsaw, @smurfix, @ethanfurman
PRs	bpo-43744: [Enum] fix issue43744 #25349

^{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 = 'https://github.com/ethanfurman'
closed_at = <Date 2021-04-15.14:03:38.746>
created_at = <Date 2021-04-06.09:16:25.127>
labels = ['type-bug', 'library', '3.10']
title = 'enum: Adding a member named _classname__ raises IndexError'
updated_at = <Date 2021-04-15.14:03:38.746>
user = 'https://github.com/smurfix'

bugs.python.org fields:

activity = <Date 2021-04-15.14:03:38.746>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = True
closed_date = <Date 2021-04-15.14:03:38.746>
closer = 'ethan.furman'
components = ['Library (Lib)']
creation = <Date 2021-04-06.09:16:25.127>
creator = 'smurfix'
dependencies = []
files = []
hgrepos = []
issue_num = 43744
keywords = ['patch']
message_count = 3.0
messages = ['390301', '390815', '391137']
nosy_count = 4.0
nosy_names = ['barry', 'smurfix', 'eli.bendersky', 'ethan.furman']
pr_nums = ['25349']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43744'
versions = ['Python 3.10']

[smurfix]

While checking out the Enum implementation I noticed that this code snippet results in an IndexError.

I have no idea which error or warning (if any) this should generate instead. Opinions?

    import enum
    class duh(enum.Enum):
        _duh__ = "moo"

[ethanfurman]

It should not be an error at all, but a False result. PR created.

[ethanfurman]

New changeset ec09973 by Ethan Furman in branch 'master':
bpo-43744: [Enum] fix _is_private (GH-25349)
ec09973