Microsoft/TypeScript

TS1518 depends on operand order in negated v-mode class unions

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#63,718 opened on Aug 4, 2026

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BugDomain: ParserHelp Wanted

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Description

🔎 Search Terms

TS1518, RegExp v flag, MayContainStrings, negated character class, ClassUnion, operand order, q{}

🕗 Version & Regression Information

Occurs in every version tested: 7.0.2 and 7.1.0-dev.20260804.1 (nightly). I searched open and closed issues.

💻 Code

const a = /[^\q{xy}b]/v; // TS1518
const b = /[^b\q{xy}]/v; // no error

🙁 Actual behavior

TS1518 only considers the first union operand. The second pattern is accepted but is invalid JavaScript.

🙂 Expected behavior

Both patterns report TS1518: MayContainStrings is true when any union operand may match a string.

Contributor guide

Research direction
Find where TS1518 is emitted in the TypeScript checker and inspect the MayContainStrings logic for character class unions. Trace how the first operand is handled and why the second operand is not considered when any union operand may match a string. Use the failing code examples to create a test case.
Tech stack
typescript
Domain
compilers
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.
1-2 days
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
GitTypeScriptRegExp
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70