~ PEG operator in the grammar

[RobEin]

BPO	46485
Nosy	@lysnikolaou, @pablogsal, @RobEin

^{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 2022-01-23.14:12:52.749>
created_at = <Date 2022-01-23.10:38:56.845>
labels = ['interpreter-core', 'invalid', '3.10']
title = '~ PEG operator in the grammar'
updated_at = <Date 2022-01-23.14:12:52.749>
user = 'https://github.com/RobEin'

bugs.python.org fields:

activity = <Date 2022-01-23.14:12:52.749>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2022-01-23.14:12:52.749>
closer = 'pablogsal'
components = ['Parser']
creation = <Date 2022-01-23.10:38:56.845>
creator = 'RobEin'
dependencies = []
files = []
hgrepos = []
issue_num = 46485
keywords = []
message_count = 3.0
messages = ['411360', '411361', '411362']
nosy_count = 3.0
nosy_names = ['lys.nikolaou', 'pablogsal', 'RobEin']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue46485'
versions = ['Python 3.10']

[RobEin]

This is probably not an error and I may misinterpret the PEP-617, but I don't know what the ~ PEG operator means in the next rule if there is no alternative:
assignment_expression:
| NAME ':=' ~ expression

And similarly with this rule if there is no more alternative:
assignment:
| NAME ':' expression ['=' annotated_rhs ]
| ('(' single_target ')'
| single_subscript_attribute_target) ':' expression ['=' annotated_rhs ]
| (star_targets '=' )+ (yield_expr | star_expressions) !'=' [TYPE_COMMENT]
| single_target augassign ~ (yield_expr | star_expressions)

Related links:
https://docs.python.org/3/reference/grammar.html
https://www.python.org/dev/peps/pep-0617/#id9

[pablogsal]

It means the same as if there is alternatives: if it fails to parse then it will immediately fail. We place them in rules like this in preparation for extension or in case there are invalid_* rules in the same set (which doesn't appear in the grammar definition).

[RobEin]

Thanks