[CP] Patterns parsing: fix ambiguity resolution for when and as.

[stereotype441]

Commit(s) to merge

637dd76

Target

Beta

Prepared changelist for beta/stable

https://dart-review.googlesource.com/c/sdk/+/299700

Issue Description

When encountering a guarded pattern of the form case IDENTIFIER when !EXPRESSION: or case IDENTIFIER.IDENTIFIER when !EXPRESSION, the parser incorrectly interprets the IDENTIFIER(s) as a type name, the when as an ordinary identifier, and the ! as a null assert pattern; then it considers the EXPRESSION part to be unexpected text and rejects it with an error.

What is the fix

The parser disambiguation rules have been adjusted so that when encountering a pattern of the form T when or T as, where T could be parsed as a valid type name, it does not try to parse the construct as a typed variable pattern (with T denoting the type and the when or as token denoting the variable name). Instead it falls back on an interpretation of the pattern where T is not a type. This could be:

• An identifier pattern or record pattern (if T begins with an identifier)
• A record pattern or parenthesized pattern (if T begins with ()

In addition to fixing the bug, this means, in effect, that typed variable patterns are no longer allowed to declare variables named when or as.

In a future release we intend to prohibit all variable and identifier patterns from declaring variables named when or as, however for this cherry-pick request, we're restricting the change to just what's necessary to fix the bug.

Why cherry-pick

Since guard expressions (the expressions after when in a guarded pattern) are boolean expressions, and ! is a boolean operator, guard expressions beginning with ! are likely to be common. Also, patterns of the form IDENTIFIER and IDENTIFIER.IDENTIFIER are likely to be comon (because these forms were both valid in switch cases prior to the introduction of patterns). So it seems likely that this issue will be triggered by user code fairly often. Without this fix, the user will see a fairly confusing parse error. The error can in principle be worked around by placing parentheses around the IDENTIFIER(s), but this is in no way obvious.

The fix is very targeted: the affected code path only triggers when a pattern is seen of the form T when or T as, where T is a sequence of tokens that could be validly parsed as a type, and the only behaviour change is to avoid ever interpreting such a token sequence as a variable declaration. It's extremely rare for a user to deliberately name a variable as (due to the meaning of as as a keyword); with the introduction of patterns it's likely that it will become equally rase for a user to deliberately name a variable when. So it's highly unlikely that a user will ever write new code in which the when or as token is expected to be interpreted as a variable name. As for existing (pre-Dart-3.0) code that is being ported to Dart 3.0, @munificent checked a big corpus of pub packages, open source Flutter apps, open source Flutter widgets, the Dart repo, and the Flutter repo, and found zero switch cases containing the identifier when (see #52199 (comment)).

Risk

low

Issue link(s)

#52199

Extra Info

No response