Microsoft/TypeScript

Expected token errors should display all expected tokens, not just the first

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#43,467 opened on Mar 31, 2021

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Effort: ModerateExperience EnhancementHelp Wanted

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Description

🕗 Version & Regression Information

Pretty sure this has been the behaviour since the rewrite.

⏯ Playground Link

Playground link with relevant code

💻 Code

function a0([a, b, [[c]]]: [number, number, string[][]]) { }
a0([1, 2, [["world"]]);

🙁 Actual behavior

On the last ), "Expected ','."

🙂 Expected behavior

On the last ), "Expected ',' or ']'."

Right now the parser only shows the first error that starts at each location. This is an OK approach, especially when parsing has gone badly wrong, but often the parser will expect multiple tokens. The parser should report all the tokens in one error that were previously issued as multiple Expected_0 errors.

Contributor guide

Research direction
Inspect the parser's error reporting logic in the TypeScript compiler source. Look for where expected token errors are generated, specifically how multiple expected tokens are aggregated. The issue suggests that only the first expected token is reported when multiple tokens could be expected at a given location. You need to find the code that handles Expected 0 errors and modify it to collect all expected tokens before generating the error message.
Tech stack
typescript
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitTypeScript
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60