TypeScript error when trying to use config in extends with typescript-eslint

[TrevorBurnham]

Consider this eslint.config.mjs:

// @ts-check
import { defineConfig } from "eslint/config";
import eslint from "@eslint/js";
import importX from "eslint-plugin-import-x";

export default defineConfig({
  ignores: ["build/"],
  extends: [
    eslint.configs.recommended,
    importX.flatConfigs.recommended // TypeScript error
  ],
});

If you have typescript-eslint@8.48.0 installed, tsc complains:

eslint.config.mjs:10:5 - error TS2322: Type 'PluginFlatConfig' is not assignable to type 'ExtendsElement'.
  Type 'PluginFlatConfig' is not assignable to type 'ConfigObject<RulesConfig>'.
    Types of property 'languageOptions' are incompatible.
      Type 'import("/project/workspace/node_modules/.pnpm/@typescript-eslint+utils@8.48.0_eslint@9.39.1_typescript@5.9.3/node_modules/@typescript-eslint/utils/dist/ts-eslint/Config").FlatConfig.LanguageOptions' is not assignable to type 'import("/project/workspace/node_modules/.pnpm/@eslint+core@0.17.0/node_modules/@eslint/core/dist/esm/types").LanguageOptions'.
        Index signature for type 'string' is missing in type 'LanguageOptions'.

10     importX.flatConfigs.typescript
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I haven't seen this issue with any other ESLint library that exports flat configs.

Here's a reproduction: https://codesandbox.io/p/devbox/objective-ganguly-h4h4v7

[TrevorBurnham]

Aha! I was able to reproduce my original issue with flatConfigs; it's just that the issue only occurs if you install a recent version of typescript-eslint, because of how it globally overrides ESLint's defineConfig types. The issue was already reported over there: typescript-eslint/typescript-eslint#11543

Reopening as I think it's worth considering updating the library's types to deal with this. I'm sure a large number of eslint-plugin-import-x's users also use typescript-eslint.

I've updated the PR description.