"Cannot find module" when switching from `tsc` to `tsgo`

[JakobJingleheimer]

Reproduction: https://github.com/JakobJingleheimer/tsgo-specifier-resolution-failure

Setup:

cd ./package/foo
npm ci

Works:

npx tsc

Doesn't work:

npx tsgo

Error:

src/main.ts:1:21 - error TS2307: Cannot find module 'helpers/add.ts' or its corresponding type declarations.

1 import { add } from 'helpers/add.ts';

Extra info:

This was encountered in a next.js app, and the repro should be sufficiently representative of the original code.

[jakebailey]

baseUrl isn't supported: https://github.com/JakobJingleheimer/tsgo-specifier-resolution-failure/blob/07deea08a563440283a1c59e57eab22ea767fa07/packages/foo/tsconfig.json#L4

This is overriding regular module resolution in tsc and allowing that non-relative resolution.

Why isn't this just a relative import? Did you mean rootDir?

(This behavior can be emulated with paths, but seems like it isn't needed anyway)

[JakobJingleheimer]

Because that's how Next.js does it.

Sooo if you remove that functionality, you'll preclude quite a lot of users.

I don't especially like those magical specifiers (I'd must rather use subpath imports), but I know many next users do. And it'd be quite a lotta work to switch from something that otherwise isn't broken.

[jakebailey]

This behavior can be implemented with a path mapping (which is much clearer in purpose):

"paths": {
    "*": "./src/*"
}

baseUrl is an artifact of the AMD days, and has not been recommended for a while; it was quite literally designed to be "this is the dir that will be hosted at the base URL of my website". https://www.typescriptlang.org/tsconfig/#baseUrl

The core problem is that we don't error when this setting is present.

[JakobJingleheimer]

Ahh, okay. Is there a migration guide somewhere? I'd be happy to contribute this to it, then you can reference the guide in the error.

Does tsgo support subpath imports? I'd rather use that because other tools understand it too.

[jakebailey]

#1381 adds a proper error message. There's no migration guide yet as these are all changes we will be putting in 6.0 too, we're just "ahead" here since it meant less to port.

Yes, package.json subpath imports work, and are certainly preferred for this sort of thing.

[JakobJingleheimer]

Nice! Maybe add Resolves https://github.com/microsoft/typescript-go/issues/1376 to the summary of #1381 so it closes this automatically with that fix?