fix: allow user to pass

Author: devjiwonchoi

packages/next/src/lib/eslint/runLintCheck.ts

@@ -135,7 +135,14 @@ async function lint(
 
     const mod = await Promise.resolve(require(deps.resolved.get('eslint')!))
 
-    const { ESLint } = mod
+    const { loadESLint } = mod
+    const ESLint = await loadESLint({
+      // To use legacy configs (.eslintrc*), user has to pass `ESLINT_USE_FLAT_CONFIG=false`.
+      // https://eslint.org/blog/2024/04/eslint-v9.0.0-released/#flat-config-is-now-the-default-and-has-some-changes
+      // The return value is `LegacyESLint`.
+      // https://github.com/eslint/eslint/blob/1def4cdfab1f067c5089df8b36242cdf912b0eb6/lib/types/index.d.ts#L1609-L1613
+      useFlatConfig: process.env.ESLINT_USE_FLAT_CONFIG !== 'false',
+    })
     let eslintVersion = ESLint?.version ?? mod.CLIEngine?.version
 
     if (!eslintVersion || semver.lt(eslintVersion, '7.0.0')) {