[syntax-errors] Detect unparenthesized assignment expressions in sets and indexes #16404
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Summary -- This PR detects another syntax error from #6591 and is stacked on #16383. This time the relaxed grammar for decorators proposed in [PEP 614](https://peps.python.org/pep-0614/) is detected for Python 3.8 and lower. The 3.8 grammar for decorators is [here](https://docs.python.org/3.8/reference/compound_stmts.html#grammar-token-decorators): ``` decorators ::= decorator+ decorator ::= "@" dotted_name ["(" [argument_list [","]] ")"] NEWLINE dotted_name ::= identifier ("." identifier)* ``` in contrast to the current grammar [here](https://docs.python.org/3/reference/compound_stmts.html#grammar-token-python-grammar-decorators) ``` decorators ::= decorator+ decorator ::= "@" assignment_expression NEWLINE assignment_expression ::= [identifier ":="] expression ``` This was the trickiest one of these to detect yet. It seemed like the best approach was to attempt to parse the old version and fall back on the new grammar if anything goes wrong, but I'm not as confident in this approach since it required adding a `Parser::try_parse_old_decorators` method. Test Plan -- New inline parser tests and linter CLI tests.
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Summary
This is another syntax error, stacked on #16386. This one detects unparenthesized assignment expressions used in set literals and comprehensions and in sequence indexes. The link to the release notes in #6591 just has this entry:
with no other information, so hopefully the test cases I came up with cover all of the changes. I also tested these out in the Python REPL and they actually worked in Python 3.9 too. I'm guessing this may be another case that was "formally made part of the language spec in Python 3.10, but usable -- and commonly used -- in Python >=3.9" as @AlexWaygood added to the body of #6591 for context managers. So we may want to change the version cutoff, but I've gone along with the release notes for now.
Test Plan
New inline parser tests and linter CLI tests.