Microsoft/TypeScript

Precedence in parsing types

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#7,094 opened on Feb 16, 2016

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 (5 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
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Description

It looks to me like this should parse, but it doesn't:

var x: number | (string)=>void;

It works with added parens:

var x: number | ((string)=>void);

But if the parser is not too hacky, then maybe it's easy to make the first work?

(FWIW, I ran into this in a rest argument, where it would be nice to avoid the extra parens in

function foo(...xs: (number | ()=>void)[]): void { }

)

Contributor guide

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
TypeScript syntaxparsing concepts
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
The TypeScript parser is in `src/compiler/parser.ts`. Look for how union types are parsed (likely in `parseUnionType` or similar) and how function types are parsed. The issue is that when a union type contains a function type on the right side, the parser incorrectly interprets the arrow without parentheses. Add logic to handle this precedence correctly without requiring extra parentheses.