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

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

[cmdcolin]

This error was happening to me when I accidentally had declare module "mymodule" but then actually was using import {MyInterface} from 'mymodule' and this caused any usage of MyInterface after that to result in the errors “Cannot use namespace ‘MyInterface’ as a type” and “Property ‘myprop’ of exported interface has or is using private name ‘MyInterface’.” on the same line

I had to delete the declare module "mymodule" line and then it worked. This was very confusing to me so I’d add a +1 for a better error message if possible

[FireStormHR]

This error was happening to me when I accidentally had declare module "mymodule" but then actually was using import {MyInterface} from 'mymodule' and this caused any usage of MyInterface after that to result in the errors “Cannot use namespace ‘MyInterface’ as a type” and “Property ‘myprop’ of exported interface has or is using private name ‘MyInterface’.” on the same line

I had to delete the declare module "mymodule" line and then it worked. This was very confusing to me so I’d add a +1 for a better error message if possible

@cmdcolin having your module declared is necessary when using typescript and having noImplicitAny turned on in the ts.config. To solve this namespace error while keeping your declaration you can put typeof in front of the place where you are using the namespace.