Microsoft/TypeScript

Bad parsing behaviour for generic arrow type arguments in class heritage / JSX open element

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#47,410 opened on Jan 12, 2022

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BugDomain: ParserHelp Wanted

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Description

Bug Report

🔎 Search Terms

shift left type arguments <<

🕗 Version & Regression Information

  • This is a crash
  • This changed between versions ______ and _______
  • This is the behavior in every version I tried, and I reviewed the FAQ for entries about ____ Generics _____
  • I was unable to test this on prior versions because _______

⏯ Playground Link

Playground link with relevant code

💻 Code

// No parse error.
(class extends f()< <T>(x: T) => T> {});
<Component< <T>(x: T) => T> />;

// Parse error.
(class extends f()<<T>(x: T) => T> {});
<Component<<T>(x: T) => T> />;

🙁 Actual behavior

Parsing error.

🙂 Expected behavior

No parse error. They are equivalent.

Additional Info

Found this issue when working on a similar fix to Babel (https://github.com/babel/babel/pull/14145). This issue is a follow-up to #23996. /cc @Kingwl since you have fixed that issue.

Contributor guide

Research direction
Study the TypeScript parser's handling of generic type arguments and JSX elements. Focus on the code path that handles '<' tokens, particularly the logic that distinguishes between generic arguments and JSX tags. Inspect the scanner for token contexts and the parser's heuristics for shift left vs. generic arrow functions.
Tech stack
typescript
Domain
tooling
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScriptGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40