Microsoft/TypeScript

Parsing issue for call expression with type arguments following left-shift

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#37,760 opened on Apr 2, 2020

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BugDomain: ParserHelp Wanted

Description

TypeScript Version: 3.8.3

Search Terms: left shift, type parameter

Expected behavior: Syntax highlighting stays intact.

Actual behavior: Syntax highlighting breaks, even with ts-ignore.

Code

function alignof<T>(): number {
  return 1;
}

// works
function foo<T>(): number {
  return 1 >> alignof<T[]>();
}

// breaks
function bar<T>(): number {
  return 1 << alignof<T[]>();
}

// still breaks, does not ignore
function baz<T>(): number {
  // @ts-ignore
  return 1 << alignof<T[]>();
}

Looks like a rescan issue on potentially ambiguous < tokens, attempting to interpret as 1 < <type>.

Playground Link: Provided

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
None
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
The bug occurs when using left shift operator (<<) followed by a function call with type arguments. The parser incorrectly rescans the < as a less than operator. The fix likely involves modifying the token rescan logic in src/compiler/scanner.ts, specifically the reScanLessThanToken function, to properly distinguish between left shift and type argument list. Reproduce the issue using the provided playground link or a local TypeScript build, then locate the rescan logic and adjust the condition to handle the < after a shift operator.