Microsoft/TypeScript

Strange "Cannot use namespace 'Foo' as a type" error with dummy `declare module "foo";` declaration

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#27,311 创建于 2018年9月24日

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 (12 评论) (18 反应) (0 负责人)TypeScript (48,455 star) (6,726 fork)batch import
BugDomain: Error MessagesHelp Wanted

描述

Maybe this is a question, but it has come up multiple times on Stack Overflow (most recently here) without a good answer, so I'm asking it here.

TypeScript Version: master (03af107)

Search Terms: "cannot use namespace" "as a type" TS2709 "declare module"

Code

declare module "foo";

declare module "bar" {
    import { Foo } from "foo";
    let x: Foo;
}

Expected behavior: An error that makes sense, or no error?

Actual behavior: error TS2709: Cannot use namespace 'Foo' as a type.

Playground Link: link

Related Issues: None found

贡献者指南

技术栈
typescript
领域
developer experiencetooling
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。
4
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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
TypeScript module systemambient declarationsnamespace behavior
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。
10
研究方向
Investigate the TypeScript checker (checker.ts) to understand why an import from an empty ambient module declaration results in the error 'Cannot use namespace 'Foo' as a type.' The module 'foo' has no exports, so the imported 'Foo' is likely treated as an export of a namespace. The fix should ensure that importing from a module with no exported members does not create a namespace. Review the relevant logic for resolving imports from ambient modules and consider whether the error message should be improved or the behavior changed. See comments on issue #27311 for more context.