Microsoft/TypeScript

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

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#27,311 opened on Sep 24, 2018

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BugDomain: Error MessagesHelp Wanted

Description

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

Contributor guide

Tech stack
typescript
Domain
developer experiencetooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
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
TypeScript module systemambient declarationsnamespace behavior
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
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.
Strange "Cannot use namespace 'Foo' as a type" error with dummy `declare module "foo";` declaration 路 Microsoft/TypeScript#27311 | Good First Issue