Emitting combined output declarations tries to resolve monorepo imports

[mischnic]

TypeScript Version: 4.0.0-dev.20200710

Search Terms: emit declaration, combined output, monorepo, outFile

Code

tsc index.ts --declaration --emitDeclarationOnly --outFile dist/index.d.ts

// packages/a/src/index.ts
import { B } from "b"; // this imports another package in the monorepo

export default function () {
	return new B();
}

// packages/b/package.json
{
	"name": "b",
	"version": "0.0.0",
	"source": "src/index.ts",
	"types": "dist/index.d.ts", // doesn't exist yet
	"main": "dist/index.js"     // doesn't exist yet
}

The files listed in the package.json of b don't exist yet because when building all packages in this monorepo setup, a is built before b.

Full repro in https://github.com/mischnic/parcel-issue-3891

Strangely enough, when omitting the --outFile flag, the declaration file for packages/a/src/index.ts (correctly) contains import ... from "b"; (this suggests that this is indeed a bug and not per design).

Expected behavior:

a/dist/index.d.ts output:

declare module "index" {
    import { B } from "b";
    export default function (): B;
}

Actual behavior:

a/dist/index.d.ts output:

declare module "index" {
    import { B } from "../../b/index"; // <---
    export default function (): B;
}

Playground Link:
https://github.com/mischnic/parcel-issue-3891

Related Issues: Not really related: #34846

[mischnic]

(I'd be happy to fix this myself if someone can confirm this and point me to the right source files)

[RyanCavanaugh]

Using outFile with modules is almost always a mistake; what are you intending to do with that setting?

[mischnic]

Thanks for the response!

We're using it in Parcel to essentially create a definitions bundle (as opposed to multiple files) for a package, e.g.

// index.ts
import { B } from "b";
export { thing } from "./other";

export default function () {
	return new B();
}
// other.ts
export function thing(){
	return "abc";
}

--tsc-->

declare module "other" {
    export function thing(): string;
}
declare module "index" {
    import { B } from "b";
    export { thing } from "other";
    export default function (): B;
}

--ast transformation-->

import { B } from "b";
export function thing(): string;
export default function (): B;

(https://github.com/parcel-bundler/parcel/blob/1f5586657f1ea8660ca1f06abdd75687851ebbb2/packages/transformers/typescript-types/src/TSTypesTransformer.js#L28-L69)

EDIT:

I just noticed that import .. from "./b" and import ... from "b" are (independent from this issue) both transpiled to "b" in the declarations output....