next lint fails when using ESLint v8 flat config

[allybee]

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/xenodochial-voice-lfgj88?workspaceId=ws_XCEs1GT1FEw6tFStZG4Cnd

To Reproduce

1. Open the terminal
2. Run linting using Next.js and observe that it fails to run, indicating there are invalid ESLint config options

$ pnpm exec next lint

Invalid Options:
- Unknown options: useEslintrc, extensions, resolvePluginsRelativeTo, rulePaths, ignorePath, reportUnusedDisableDirectives
- 'extensions' has been removed.
- 'resolvePluginsRelativeTo' has been removed.
- 'ignorePath' has been removed.
- 'rulePaths' has been removed. Please define your rules using plugins.
- 'reportUnusedDisableDirectives' has been removed. Please use the 'overrideConfig.linterOptions.reportUnusedDisableDirectives' option instead.

3. Run linting using ESLint and observe that it runs successfully

$ pnpm exec eslint .

Current vs. Expected behavior

Current behavior

next lint does not work with an ESLint flat config (e.g., eslint.config.js) on eslint@^8.21.0.
It does work successfully with eslint@^9.

Expected behavior

It is expected that next lint runs successfully with a valid ESLint flat config on eslint@^8 and eslint@^9.

ESLint introduced the flat config in v8.21.0 and it is the default format in v9.x.
Although ESLint v8.x has reached end-of-life, some widely used shared configs have not yet migrated to v9.x (e.g., eslint-config-airbnb). Flat config compatibility with next lint and ESLint v8.x allows codebases tied to ESLint v8.x to avoid or move away from the deprecated eslintrc format.

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Sun Aug  6 20:05:33 UTC 2023
  Available memory (MB): 4242
  Available CPU cores: 2
Binaries:
  Node: 20.9.0
  npm: 9.8.1
  Yarn: 1.22.19
  pnpm: 8.10.2
Relevant Packages:
  next: 15.2.0-canary.25 // Latest available version is detected (15.2.0-canary.25).
  eslint: 8.57.1
  eslint-config-next: 15.1.6
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.3.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Linting

Which stage(s) are affected? (Select all that apply)

next build (local)

Additional context

I believe the culprit is the following ESLint v9.x version check:

https://github.com/vercel/next.js/blob/v15.2.0-canary.25/packages/next/src/lib/eslint/runLintCheck.ts#L178-L193

Patching it to support ESLint versions greater than v8.21.0 worked in my case:

-        if (_semver.default.gte(eslintVersion, '9.0.0') && useFlatConfig) {
+        if (_semver.default.gte(eslintVersion, '8.21.0') && useFlatConfig) {
             for (const option of [
                 'useEslintrc',
                 'extensions',

[samcx]

@allybee Thank you for sharing! Taking a look 👁