Various errors under `exactOptionalPropertyTypes`

[LukeAbby]

Describe the bug

If you have exactOptionalPropertyTypes enabled then various parts of vitest do not compile.

There are 249 errors inside the repo if you compile with it but what's more important is that it's observable for users.

I'd be happy to contribute these fixes but I want to know if I should limit to what users can observe (e.g. errors within the output .d.ts files) or fix it even within function bodies.

Reproduction

This is personally how I ran into it:

import type { VitestRunner } from "@vitest/runner"; // types: v4.0.14
import { VitestTestRunner } from "vitest/runners"; // types: v4.0.14

export default class Runner extends VitestTestRunner implements VitestRunner {}
//                   ^ Class 'Runner' incorrectly implements interface 'VitestRunner'.
//                       The types of 'config.name' are incompatible between these types.
//                         Type 'string | undefined' is not assignable to type 'string'.
//                           Type 'undefined' is not assignable to type 'string'.

https://www.typescriptlang.org/play/?exactOptionalPropertyTypes=true#code/JYWwDg9gTgLgBDAnmApnA3nAasGKDOMASgK4B2ZKUcAvnAGZQQhwBEAAgG64EwD0UcpSisA3HD58EyAgC44nACwA6AAzKAjIoBQoSLAzYehACq9SFKrQZMWrbnkIChVfGIlSkqfPKVrNOtooAB768AAmKPQAhiQANvAAxnHR+PhwFsJwIXhk4ek4jjBmhJlWenEoIChkMAXGxC7U6DTaknAdnV3dXQB6cADCKWlwAORlUKNwwGSJ0FAoiTBxiNPgldW16TN4UDGJaKOF5k2jym1SPVfXcCYAFmheBHAQ9GNzZPTAAObKZNHVKbRBbTWbMMDRGDAABGlTg0JQMAA7igaggHvhHjJ8Od2jd8d0TDIxoQoDNvnAAD5wciRL6UcJTYDpMgQeCpfA-f6wx4QaSoEkwMlkb5nC4EiVdIkC0a0qIzFCM6YstlwDlc6I8hB8p6C4Wi5RAA

System Info

System:
    OS: Linux 6.6 Ubuntu 24.04.3 LTS 24.04.3 LTS (Noble Numbat)
    CPU: (16) x64 AMD Ryzen 7 7840HS w/ Radeon 780M Graphics
    Memory: 3.88 GB / 27.31 GB
    Container: Yes
    Shell: 5.2.21 - /bin/bash
  Binaries:
    Node: 24.5.0 - /home/david/.nvm/versions/node/v24.5.0/bin/node
    npm: 11.5.1 - /home/david/.nvm/versions/node/v24.5.0/bin/npm
    bun: 1.2.4 - /home/david/.bun/bin/bun
    Deno: 2.1.1 - /home/david/.deno/bin/deno
  npmPackages:
    @vitest/browser-playwright: ^4.0.13 => 4.0.13 
    @vitest/ui: ^4.0.13 => 4.0.13 
    vite: ^7.2.4 => 7.2.4 
    vitest: ^4.0.13 => 4.0.13

Used Package Manager

npm

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[hi-ogawa]

I'd be happy to contribute these fixes but I want to know if I should limit to what users can observe (e.g. errors within the output .d.ts files) or fix it even within function bodies.

Does this error for "normal" testing code or you only noticed when using advanced API? We do have test to make sure skipLibCheck: false with Vitest pass such as https://github.com/vitest-dev/vitest/tree/main/test/dts-fixture and exactOptionalPropertyTypes didn't seem to fail for this case.

---

I think if it's only with advanced custom runner API, we might worry less. Then if it's about internal code to adopt to exactOptionalPropertyTypes, we worry less further.

[LukeAbby]

I noticed it for a custom runner, yeah. I think that qualifies under the "advanced API"?

As aforementioned you can see it anytime you extend VitestTestRunner and implement VitestRunner as the documentation recommends though:

import type { VitestRunner } from "@vitest/runner"; // types: v4.0.14
import { VitestTestRunner } from "vitest/runners"; // types: v4.0.14

export default class Runner extends VitestTestRunner implements VitestRunner {}
//                   ^ Class 'Runner' incorrectly implements interface 'VitestRunner'.
//                       The types of 'config.name' are incompatible between these types.
//                         Type 'string | undefined' is not assignable to type 'string'.
//                           Type 'undefined' is not assignable to type 'string'.

[hi-ogawa]

Oh btw I guess this wasn't really about skipLibCheck: false. Personally, I haven't had a good experience with exactOptionalPropertyTypes, but if you want to fix that part only somehow, I think we'll likely accept the PR.

[Bertie690]

I also get errors on extending a custom reporter - attempting to pass the very same File interface that is exported by @vitest/runner proceeds to cause TypeScript to freak the heck out.

import type { File } from "@vitest/runner";
import { DefaultReporter } from "vitest/reporters";

export default class DummyReporter extends DefaultReporter {
  override reportTestSummary(files: File[], errors: unknown[]): void {
    // Errors about `File` not being assignable to... itself???
    // This is presumably due to all the fields being `x?: XYZ` instead of `x?: XYZ | undefined`
    super.reportTestSummary(files, errors);
  }
}