PEG discrepancy on 'if {x} {a}: pass'

[gvanrossum]

BPO	41659
Nosy	@gvanrossum, @terryjreedy, @lysnikolaou
PRs	bpo-41659: Disallow curly brace directly after primary #22996 [3.9] bpo-41659: Disallow curly brace directly after primary (GH-22996) #23006

^{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/lysnikolaou'
closed_at = <Date 2020-10-27.22:39:15.328>
created_at = <Date 2020-08-28.21:43:15.737>
labels = ['type-bug', '3.9', '3.10']
title = "PEG discrepancy on 'if {x} {a}: pass'"
updated_at = <Date 2020-10-27.22:39:15.328>
user = 'https://github.com/gvanrossum'

bugs.python.org fields:

activity = <Date 2020-10-27.22:39:15.328>
actor = 'lys.nikolaou'
assignee = 'lys.nikolaou'
closed = True
closed_date = <Date 2020-10-27.22:39:15.328>
closer = 'lys.nikolaou'
components = []
creation = <Date 2020-08-28.21:43:15.737>
creator = 'gvanrossum'
dependencies = []
files = []
hgrepos = []
issue_num = 41659
keywords = ['patch']
message_count = 5.0
messages = ['376048', '376065', '376412', '379792', '379809']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'terry.reedy', 'lys.nikolaou']
pr_nums = ['22996', '23006']
priority = 'low'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41659'
versions = ['Python 3.9', 'Python 3.10']

[gvanrossum]

I just noticed a subtle discrepancy between the old parser and the PEG parser.

Consider this syntax error:

if x {a}: pass

The old parser places the caret at the '{':

$ python3.8 -c 'if x { a } : pass'
  File "<string>", line 1
    if x { a } : pass
         ^
SyntaxError: invalid syntax

The PEG parser puts it at 'a':

$ python3.10 -c 'if x { a } : pass'
  File "<string>", line 1
    if x { a } : pass
           ^
SyntaxError: invalid syntax

I don't think we should put much effort into fixing it -- it's just a curiosity. I suspect it's got to do with some lookahead.