Microsoft/TypeScript

Double error message when referencing variable of type union of string literals from ambient declaration

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#63050 opened on Jan 25, 2026

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 (3 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
Domain: Error MessagesFix AvailableHelp WantedPossible Improvement

Description

🔎 Search Terms

double error, same message twice, ambient declaration, string literal union

🕗 Version & Regression Information

This changed between versions 3.9.7 and 4.0.5

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.3#code/CYUwxgNghgTiAEYD2A7AzgF3gDwFzwCIoD4AfQgIwIChrl0sBPfFAVwFsKQZ4BeHIA

💻 Code

declare const x: "a" | "b"

const y: number = x
//    ~
//    Type 'string' is not assignable to type 'number'.
//      Type 'string' is not assignable to type 'number'. (2322)

🙁 Actual behavior

You see same error message twice

🙂 Expected behavior

The error message should be the one in 3.9.7 ie...

Type '"a" | "b"' is not assignable to type 'number'.
  Type '"a"' is not assignable to type 'number'. (2322)

Or the following message just once also works...

Type 'string' is not assignable to type 'number'. (2322)

Additional information about the issue

No response

Contributor guide

Tech stack
typescript
Domain
backenddeveloper experience
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.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
basic TypeScriptunderstanding of error messages
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Investigate the TypeScript compiler's error reporting logic, specifically in the checker.ts file where type checking of unions occurs. The issue is a regression between v3.9.7 and v4.0.5, so compare error emission in those versions. Look at how 'Type 'string' is not assignable to type 'number'' is generated twice instead of the expected detailed message. A git bisect between these versions may pinpoint the exact commit. Also check the playground link for repro.