`extends` + `files` causes rules to be dropped

[kevva]

Since v1.39.0 (PR #323), configs with nested array file patterns (AND globs) are skipped by processConfigFiles. While this fixed the invalid config output from #322, it silently drops all rules from any ESLint config that uses extends inside a config object with files because ESLint's defineConfig turns that combination into AND-pattern nested arrays.

This is a very common pattern in ESLint flat configs:

import { defineConfig } from 'eslint/config'
import tseslint from 'typescript-eslint'
import react from 'eslint-plugin-react'

export default defineConfig([
  // Rules from tseslint are silently dropped by migrate
  { files: ['**/*.ts'], extends: [tseslint.configs.recommended] },
  // Rules from react are silently dropped by migrate
  { files: ['**/*.{jsx,tsx}'], extends: [react.configs.flat.recommended] },
])

ESLint's defineConfig resolves extends by creating AND-pattern file arrays (e.g., [["**/*.ts", "**/*.{js,ts}"]]). processConfigFiles filters these out, returning an empty array, and buildConfig then skips the entire config via if (validFiles.length === 0) continue.

Reproduction:

// eslint.config.js
import { defineConfig } from 'eslint/config'
import tseslint from 'typescript-eslint'

export default defineConfig([
  {
    files: ['**/*.ts'],
    extends: [tseslint.configs.recommended],
    rules: { '@typescript-eslint/no-explicit-any': 'error' },
  },
])

Run npx @oxlint/migrate --type-aware eslint.config.js and you'll see that the typescript-eslint rules will be missing from the output.

[Sysix]

I tried to optimize to allow file array with the same values: ['**/*.vue', '**/*.vue'].
Sadly, we can not support more complex one like: ['**/*.{vue}', '**/*.vue'], which should technically be the same.

Because oxlint does not support this file overrides with AND glob syntax, the safest way is to skip them.
Do you have another suggestion?

[kevva]

@Sysix, thanks for looking into this issue so fast! Maybe we can support the pattern that is the most narrow/restrictive? So ['**/*.{ts,tsx}', '**/*.{js,ts,jsx,tsx}'] would become ['**/*.{ts,tsx}'].