Microsoft/TypeScript

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

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#63,050 创建于 2026年1月25日

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 (3 评论) (0 反应) (0 负责人)TypeScript (48,455 star) (6,726 fork)batch import
Domain: Error MessagesFix AvailableHelp WantedPossible Improvement

描述

🔎 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

贡献者指南

技术栈
typescript
领域
backenddeveloper experience
议题类型
bug
难度面向新贡献者的预计实现难度，1 表示很小改动，5 表示专家级工作。
3
预计时间有经验贡献者完成调查、实现、测试并准备 pull request 的粗略时间范围。
1-2 days
活动状态议题当前的可参与程度：新鲜、活跃、陈旧、阻塞或等待维护者输入。
fresh
清晰度议题是否清楚说明期望改动、验收标准和下一步。
clear
前置要求
basic TypeScriptunderstanding of error messages
新手友好度1-100 的估计分数，表示该议题对首次贡献者的友好程度。
40
研究方向
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.