fix(types): narrow down the return type of defineConfig

Fixes compatibility with `mergeConfig`, as reported in vuejs/create-vue#313

Before the change, `defineConfig` would return `UserConfigExport`, which
is a union type. This would cause `mergeConfig` to fail, as it doesn't
accept the `UserConfigFn` type in the union. (vitejs#13135)

After the change, `defineConfig` returns the same type that it receives,
so it would exclude `UserConfigFn` from the return type whenever
possible.

packages/vite/src/node/__tests__/config.spec.ts

@@ -7,7 +7,7 @@ import { mergeConfig } from '../publicUtils'
 
 describe('mergeConfig', () => {
   test('handles configs with different alias schemas', () => {
-    const baseConfig: UserConfigExport = {
+    const baseConfig = defineConfig({
       resolve: {
         alias: [
           {
@@ -16,16 +16,16 @@ describe('mergeConfig', () => {
           },
         ],
       },
-    }
+    })
 
-    const newConfig: UserConfigExport = {
+    const newConfig = defineConfig({
       resolve: {
         alias: {
           bar: 'bar-value',
           baz: 'baz-value',
         },
       },
-    }
+    })
 
     const mergedConfig: UserConfigExport = {
       resolve: {

packages/vite/src/node/config.ts

@@ -109,7 +109,7 @@ export type UserConfigExport = UserConfig | Promise<UserConfig> | UserConfigFn
  * The function receives a {@link ConfigEnv} object that exposes two properties:
  * `command` (either `'build'` or `'serve'`), and `mode`.
  */
-export function defineConfig(config: UserConfigExport): UserConfigExport {
+export function defineConfig<T extends UserConfigExport>(config: T): T {
   return config
 }