Type incompatibilities between mergeConfig and defineConfig when the latter is used with a function

[vinassefranche]

Describe the bug

I am creating a config with defineConfig by providing a function. When I want to use this function in mergeConfig, I get a TypeScript error:
Argument of type 'UserConfigExport' is not assignable to parameter of type 'UserConfig | Promise<UserConfig>'. Type 'UserConfigFnObject' is not assignable to type 'UserConfig | Promise<UserConfig>'.

Here's the code that's in the typescript playground added as a reproduction. The function in defineConfig is a dummy function here to showcase the issue. I know that here, I could get rid of the function but in my real repository, I need to access the command information

import { defineConfig, mergeConfig } from 'vite';

// Does not work when using a function parameter
const config = defineConfig(() => ({
    plugins: [],
}));
    
mergeConfig(
  config,
  defineConfig({
    envPrefix: 'PUBLIC_',
  }),
);

// Works when not using a function parameter
const config2 = defineConfig({
    plugins: [],
});

mergeConfig(
  config2,
  defineConfig({
    envPrefix: 'PUBLIC_',
  }),
);

The issue is that mergeConfig does not accept functions (see D extends Function ? never : D, in

vite/packages/vite/src/node/utils.ts

Line 1066 in a48bf88

export function mergeConfig<

) so UserConfigExport is not an acceptable type for it.

Is there something I'm doing wrong or is defineConfig not defined as it should be?

Reproduction

https://www.typescriptlang.org/play#code/JYWwDg9gTgLgBAbzgEwKYDNgDtUGEJaYDmANHCKlEXgcXAL5zpQQhwDkAbsDKuwNwAoQQHoRcACIRUAZzhYI8AO7QA1nCUALVFjgBXGdiJwAhkz1YAxjGAE4YE1BMVeUQZYIz4HwsGMBeFAxsGl8iAApwgEo4fwA+OHCEQThU+wAbPSJsGQAuOABtAF0SQXooqKE0uEEKKlDicJS4H2JS1LRMHHwwpObUnU4ABShggA989iGAVQAhABkASVwAfXZ2hijSyuExOAB1NTktHXlFfUMsYzN0C2tbXQcnF0p3T29aPwAmWKCuhr8fWqYEy2SweUKJTKO1qlGoPUazVa3w2nRCCMByWqgxG40mMwWyzWG3K234QA

Steps to reproduce

No response

System Info

vitest@4.4.7

Used Package Manager

yarn

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to vuejs/core instead.
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• The provided reproduction is a minimal reproducible example of the bug.

[patak-dev]

This is expected. See #13135 (and the linked PR from it). You can call the config function before passing it to mergeConfig.

[vinassefranche]

Thanks for your answer!

I'm not sure I understand the workaround, to be honest. I have 2 issues:

1. config is not callable
   I can't just call config as Typescript prevents me from calling it (rightfully, as it's a union type of types that are not all functions)
   Maybe defineConfig should have more overloads to be able to discriminate more the result type.
   Why not use a generic for this, by the way?

   export function defineConfig<Config extends UserConfigExport>(config: Config): Config {
     return config
   }

2. Even if config was callable, I'm supposed to pass an env parameter to it, right?
   Should I use mergeConfig in a definedConfig like this?

   defineConfig(env => mergeConfig(
     config(env),
     defineConfig({
       envPrefix: 'PUBLIC_',
     }),
   ));

Thanks again for your interest in this. Tell me if I'm not clear in what I'm asking!

[patak-dev]

Let's reopen the issue to keep discussing this. I agree it would be good if defineConfig would return a function type when passing a function. PR welcome to check how things would look when we run CI against other projects in the ecosystem.

And if that is the case, your code snippet for 2 seems good to me. You can cast the function for now after the first defineConfig so you aren't blocked.

[vinassefranche]

I've opened the pull request: #13958
I see that there's a defineConfig in the types of vitest with the old typings (the one before the pull request you referenced in your first post) Do you know if I need to open a pull request there too or if it's automatically back-ported from this repository?

[patak-dev]

Thanks! There is no automatic process to port things to Vitest, but you could first wait until #13958 is cleared.

[Anoesj]

Would be nice to update the documentation with the latest info. I'm glad I found this issue, otherwise I wouldn't have figured out that I could use this pattern:

import { mergeConfig, defineConfig } from 'vite';
import viteConfig from './vite.config';

export default defineConfig((env) => mergeConfig(
  viteConfig(env),
  defineConfig({
    test: {
      // vitest config
    },
  }),
));

[patak-dev]

@Anoesj would you like to send a PR to the Vitest repo to do that? That is a good contribution for the project

[vinassefranche]

@patak-dev I just created this pull request to add it to vite's documentation as well. Tell me if you don't think it's a good idea.