fix(types): improve defineConfig callback inference

[Vishnuuuu24]

Summary

• improve defineConfig callback overload inference so sync/async callback configs preserve literal option types
• fix callback config cases where literals like build.minify: 'terser' were widened and triggered TS2769
• add dts regression tests for callback config minify literals without as const

Details

• update defineConfig overloads in packages/vite/src/node/config.ts to use a callback-first generic signature:
  • defineConfig<T extends UserConfigFn>(config: T & UserConfigFn): ...
• keep return typing behavior aligned with existing UserConfigFnObject / UserConfigFnPromise / UserConfigFn expectations
• add tests in packages/vite/src/node/__tests_dts__/config.ts:
  • sync callback with build.minify: 'oxc'
  • async callback with build.minify: 'terser'
  • invalid nested build.unknown still errors (@ts-expect-error)

Validation

• pnpm run build-types-check
• pnpm run typecheck

Refs #21684

[Copilot]

Pull request overview

This PR fixes a TypeScript type inference issue where using defineConfig with async callbacks caused literal option types (like build.minify: 'terser') to be widened to string, resulting in TS2769 errors.

Changes:

• Refactored defineConfig overloads to use a generic callback-first signature that preserves literal types
• Added comprehensive dts regression tests for sync/async callbacks with minify literals

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/vite/src/node/config.ts	Replaced three function overloads with a single generic overload that preserves literal types using conditional return types
packages/vite/src/node/tests_dts/config.ts	Added three test cases validating literal type preservation in sync/async callbacks and invalid option detection

[Vishnuuuu24]

Thanks for calling this out. You’re right, I used AI assistance while preparing this PR and parts ended up too close to #21698, including test shape and the proxy-bypass point you had already identified. I should have explicitly credited your prior work. Sorry about that, and thanks for the review.

[Vishnuuuu24]

Thanks for pointing this out. I used AI assistance while preparing this PR, and the result ended up too close to your draft in #21698 (including the same test shape and the proxy-bypass workaround you had already called out). I should have explicitly referenced your prior work. Sorry about that, and thanks for the review, will make the required changes soon.

…

On Tue, 24 Feb 2026 at 18:27, 翠 ***@***.***> wrote: ***@***.**** commented on this pull request. ------------------------------ In packages/vite/src/node/__tests_dts__/config.ts <#21699 (comment)>: > @@ -37,6 +37,28 @@ defineConfig({ unknownProperty: 1, }) +defineConfig(() => ({ + base: '', + build: { + minify: 'oxc', + }, +})) + +defineConfig(async () => ({ + base: '', + build: { + minify: 'terser', + }, +})) + +// @ts-expect-error --- nested invalid option `build.unknown` should error I suspect you copied this part of code from my PR <#21698>. It's too similar to believe that it's a coincidence. If that's the case, I'd suggest at least mentioning it. ------------------------------ In playground/proxy-bypass/vite.config.js <#21699 (comment)>: > @@ -9,7 +9,7 @@ export default defineConfig({ '/nonExistentApp': { target: 'http://localhost:9607', bypass: () => { - return false + return /** @type {false} */ (false) This is the breakage I mentioned at #21698 (comment) <#21698 (comment)> — Reply to this email directly, view it on GitHub <#21699 (review)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/BAXYJBQVEWT4QZUU4EYYLB34NRDCLAVCNFSM6AAAAACV5ZPKNGVHI2DSMVQWIX3LMV43YUDVNRWFEZLROVSXG5CSMV3GSZLXHMZTQNBXG4YTSNZVGY> . You are receiving this because you authored the thread.Message ID: ***@***.***>