[CP] [parser] Fix issue 52954 about nested record destructuring with shorthand

[stereotype441]

Commit(s) to merge

a445fae

Target

Stable

Prepared changelist for beta/stable

https://dart-review.googlesource.com/c/sdk/+/322587

Issue Description

When a record pattern is nested inside another record pattern, and the inner record pattern uses "shorthand" syntax, the parser sometimes becomes confused and reports syntax errors. Examples include:

final ((:a, :b), c) = record;
final (c, (:a, :b)) = record;

The problem affects all platforms, including the analyzer.

What is the fix

The solution is to modify the parsing method parseParenthesizedPatternOrRecordPattern, which was using an overly general rule to accept named record fields. Technically a named record field should take the form identifier ':' pattern, but the parser was allowing the identifier to be any token for error recovery purposes. This meant that when parsing a pattern like ((:a, :b), c), instead of recognizing (:a, :b) as a nested record pattern, it was assuming it was a malformed named record field, with the ( being the malformed name.

The fix excludes ( from being considered as a record field name, even during error recovery, so that (:a, :b) is properly recognized as an unnamed field.

Why cherry-pick

Without this fix, any user trying to pattern match a record pattern containing a nested record pattern, where the nested record pattern begins with (:, will see obscure and confusing syntax errors using Dart 3.1. Any user doing this sort of pattern match using Dart 3.2 will get correct behavior, but if they publish their code on pub, it won't work with Dart 3.1.

The fix is very low risk, since it only affects an error recovery path.

Risk

low

Issue link(s)

#52954

Extra Info

No response

[itsjustkevin]

@leafpetersen and @jakemac53 could you take a look at this cherry-pick request?

[leafpetersen]

LGTM