fix(mergeConfig) Don't accept callback form config

mergeConfig function doesn't work with config in
callback form. To make the error more transparent
added both a runtime and type checks.

docs/guide/api-javascript.md

@@ -266,6 +266,10 @@ function mergeConfig(
 
 Deeply merge two Vite configs. `isRoot` represents the level within the Vite config which is being merged. For example, set `false` if you're merging two `build` options.
 
+::: tip NOTE
+`mergeConfig` accepts only config in object form. If you have a config in callback form, you should call it before passing into `mergeConfig`.
+:::
+
 ## `searchForWorkspaceRoot`
 
 **Type Signature:**

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

@@ -1,7 +1,7 @@
 import { describe, expect, test } from 'vitest'
 import type { InlineConfig } from '..'
 import type { PluginOption, UserConfig, UserConfigExport } from '../config'
-import { resolveConfig } from '../config'
+import { defineConfig, resolveConfig } from '../config'
 import { resolveEnvPrefix } from '../env'
 import { mergeConfig } from '../publicUtils'
 
@@ -184,6 +184,27 @@ describe('mergeConfig', () => {
     expect(mergeConfig(baseConfig, newConfig)).toEqual(mergedConfig)
     expect(mergeConfig(newConfig, baseConfig)).toEqual(mergedConfig)
   })
+
+  test('throws error with functions', () => {
+    const baseConfig = defineConfig(() => ({ base: 'base' }))
+    const newConfig = defineConfig(() => ({ base: 'new' }))
+
+    expect(() =>
+      mergeConfig(
+        // @ts-expect-error TypeScript shouldn't give you to pass a function as argument
+        baseConfig,
+        newConfig,
+      ),
+    ).toThrowError('Cannot merge config in form of callback')
+
+    expect(() =>
+      mergeConfig(
+        {},
+        // @ts-expect-error TypeScript shouldn't give you to pass a function as argument
+        newConfig,
+      ),
+    ).toThrowError('Cannot merge config in form of callback')
+  })
 })
 
 describe('resolveEnvPrefix', () => {

packages/vite/src/node/utils.ts

@@ -1037,11 +1037,18 @@ function mergeConfigRecursively(
   return merged
 }
 
-export function mergeConfig(
-  defaults: Record<string, any>,
-  overrides: Record<string, any>,
+export function mergeConfig<
+  D extends Record<string, any>,
+  O extends Record<string, any>,
+>(
+  defaults: D extends Function ? never : D,
+  overrides: O extends Function ? never : O,
   isRoot = true,
 ): Record<string, any> {
+  if (typeof defaults === 'function' || typeof overrides === 'function') {
+    throw new Error(`Cannot merge config in form of callback`)
+  }
+
   return mergeConfigRecursively(defaults, overrides, isRoot ? '' : '.')
 }