PEG parser doesn't accept extended unpacking in with statement

[oremanj]

BPO	41979
Nosy	@gvanrossum, @benjaminp, @pablogsal, @miss-islington, @isidentical, @oremanj
PRs	bpo-41979: Accept star-unpacking on with-item targets #22611 [3.9] bpo-41979: Accept star-unpacking on with-item targets (GH-22611) #22612

^{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 2020-10-09.09:58:18.249>
created_at = <Date 2020-10-09.06:56:54.326>
labels = ['interpreter-core', '3.9']
title = "PEG parser doesn't accept extended unpacking in with statement"
updated_at = <Date 2020-10-09.10:31:15.580>
user = 'https://github.com/oremanj'

bugs.python.org fields:

activity = <Date 2020-10-09.10:31:15.580>
actor = 'miss-islington'
assignee = 'none'
closed = True
closed_date = <Date 2020-10-09.09:58:18.249>
closer = 'lys.nikolaou'
components = ['Interpreter Core']
creation = <Date 2020-10-09.06:56:54.326>
creator = 'Joshua Oreman'
dependencies = []
files = []
hgrepos = []
issue_num = 41979
keywords = ['patch']
message_count = 3.0
messages = ['378305', '378309', '378311']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'benjamin.peterson', 'pablogsal', 'miss-islington', 'BTaskaya', 'Joshua Oreman']
pr_nums = ['22611', '22612']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue41979'
versions = ['Python 3.9']

[oremanj]

On Python 3.9.0 with the new PEG parser, the following statement produces a SyntaxError:

with contextlib.nullcontext(range(1, 5)) as (first, *rest, last):
    print(first, rest, last)

On 3.8.x, or 3.9.0 with -X oldparser, it succeeds and prints "1 [2, 3] 4" as expected.

As I understand it, the thing after 'as' is an assignment target and should accept anything that can go on the LHS of an equals sign.

[pablogsal]

New changeset 48f305f by Batuhan Taskaya in branch 'master':
bpo-41979: Accept star-unpacking on with-item targets (GH-22611)
48f305f

[miss-islington]

New changeset 42157b9 by Batuhan Taskaya in branch '3.9':
[3.9] bpo-41979: Accept star-unpacking on with-item targets (GH-22611) (GH-22612)
42157b9