Pulling in ambient types through triple slash directive stopped working when exports field present on package.json

[dummdidumm]

Bug Report

🔎 Search Terms

tripleslash ambient types

🕗 Version & Regression Information

• This changed between the latest version o 4.4 and 4.5.2

⏯ Playground Link

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💻 Code

global.d.ts

/// <reference types="svelte" />

🙁 Actual behavior

Since 4.5 referencing types via triple slash directives in an ambient module to pull in other ambient types into the global scope stopped working when the package whose types should be pulled in has exports defined in their package.json, but no types field inside exports.

🙂 Expected behavior

Referencing types via triple slash directives should pull in their ambient types into the global scope

Steps to reproduce

This is reproducible using the Svelte starter template which uses this technique to tell TypeScript to treat any file import ending with .svelte as valid, resolving it to an ambient module named *.svelte.

1. Clone https://github.com/sveltejs/template, for example through npx degit sveltejs/template svelte-app
2. cd svelte-app/ && node scripts/setupTypeScript.js - this converts the template to using TS
3. In the package.json pin the svelte version to 3.44.1 (the last version where this occurs, we since added a fix on our side, but I still think this a TS bug)
4. npm install
5. Open project in VS Code and set TS to use the workspace TS version
6. Open main.ts --> bug: import App from './Apps.svelte'; has an error because the triple slash directive in global.d.ts doesn't work

If I change the triple slash directive in global.d.ts to /// <reference path="../node_modules/svelte/types/runtime/index.d.ts" /> it works. That path reference is what the types field in the package.json of Svelte resolves to, so I would expect /// <reference types="svelte" /> to work.

My guess is that this is a side effect of the new Node12 mode which was part of the 4.5 beta releases but was then pulled out of the final release. With that new mode it would be expected to have a corresponding types entry as part of the exports in Svelte's package.json, but since that new resolution isn't used I would expect the types to get resolved like in 4.4 and downwards. In fact, I just tested this out and it indeed works in 4.5 using /// <reference types="svelte" /> if types is part of the exports in package.json.

I guess this not only impacts the Svelte starter template but every package which has exports without types in it and wants to support loading definitions into the global scope through tripleslash directives.