--isolatedModules should warn about re-export of type

[ghost]

TypeScript Version: nightly (Version 2.3.0-dev.20170417)

Code

a.ts

export const A = 0;
export type B = number;

b.ts

export { A, B } from "./a";

Expected behavior:

An error saying that the emit of b.ts depends on the contents of a.ts.

Actual behavior:

b.js is export { A } from "./a";.

[Jessidhia]

Hm... what exactly is --isolatedModules supposed to do?

I have been using it as a way of requiring that only files parsed as "module" would be considered valid (i.e. requires at least one "import" or "export" in typescript, forbids (potentially sloppy) scripts), though preventing reexporting of types would break things on my project.

I can turn of --isolatedModules off in 2.4 in that case, but since it seems to do more than what I thought it did, I wonder what would be the side effects.

[mhegazy]

It guarantees that the file can be transpiled in isolation. Hence the isolated part. Things that need cross file information to emit like namespaces spanning multiple files or const enums are not alloewd. This is what you would use if you were using transpileModule API.

[Jessidhia]

So I assume the absence of --isolatedModules is what causes this behavior? DefinitelyTyped/DefinitelyTyped#16118 (comment)

If that is the case, disabling --isolatedModules could be problematic in my codebase, as I have multiple independent (non-exported) interface Props {} in different files. IIUC, this would cause them to augment each other instead of still being independent.

[ghost]

--isolatedModules won't cause "isolated" type-checking behavior, just isolated transpilation. So multiple globally-declared interfaces will still be merged. Not sure which specific issue you're talking about as the link there just takes me to the issue, not to a particular comment.

[Jessidhia]

@Andy-MS ah, that happens sometimes in github. The thing in question was about type / interface in .d.ts being exported regardless of the presence of export, which upon thinking further wouldn't (unintentionally) be further augmentable.