enable type checking for eslint.config.mjs again

there is conflict of type definitions between `@types/eslint` and
`typescript-eslint`. These packages provide their own types for
a config object in `eslint.config.mjs` and the types are not assignable.
`eslint-plugin-n` uses `@types/eslint` but `ts.config()` does not accept
the flat config object of the plugin.  It seems that the maintainer of
`typescript-eslint` won't improve this situation.

typescript-eslint/typescript-eslint#8613 (comment)

eslint.config.mjs

@@ -8,10 +8,17 @@ import n from 'eslint-plugin-n';
 export default ts.config(
     eslint.configs.recommended,
     ...ts.configs.recommendedTypeChecked,
+    // @ts-expect-error Types of typescript-eslint is not compatible with @types/eslint.
+    // These packages provide their own types for eslint.config.mjs and they are not compatible with each other.
+    // eslint-plugin-n uses @types/eslint but `ts.config()` does not accept the flat config of the plugin.
+    // The maintainer of typescript-eslint won't improve this situation. So ignoring type error here is the best
+    // we can do.
+    // See https://github.com/typescript-eslint/typescript-eslint/issues/8613#issuecomment-1983488262
     n.configs['flat/recommended'],
     {
         languageOptions: {
             parserOptions: {
+                projectService: true,
                 project: 'tsconfig.json',
             },
         },
@@ -99,4 +106,17 @@ export default ts.config(
             'mocha/no-top-level-hooks': 'error',
         },
     },
+    {
+        files: ['eslint.config.mjs'],
+        languageOptions: {
+            parserOptions: {
+                projectService: false,
+                project: 'tsconfig.eslint.json',
+            },
+        },
+        rules: {
+            '@typescript-eslint/naming-convention': 'off',
+            'n/no-extraneous-import': 'off',
+        },
+    },
 );

package.json

@@ -13,9 +13,10 @@
     "watch:tsc": "tsc -p . --watch --preserveWatchOutput --pretty",
     "watch:mocha": "mocha --watch-files \"./test/*.js\"",
     "watch": "concurrently -c auto npm:watch:tsc npm:watch:mocha",
-    "lint:eslint": "eslint --max-warnings 0 \"./**/*.ts\"",
+    "lint:eslint": "eslint --max-warnings 0 \"./**/*.ts\" eslint.config.mjs",
+    "lint:tsc-eslint": "tsc -p tsconfig.eslint.json --pretty",
     "lint:prettier": "prettier --check \"./**/*.ts\" \"./**/*.mjs\"",
-    "lint": "concurrently -c auto npm:lint:eslint npm:lint:prettier",
+    "lint": "concurrently -c auto npm:lint:eslint npm:lint:prettier npm:lint:tsc-eslint",
     "fix:eslint": "eslint --fix \"./**/*.ts\"",
     "fix:prettier": "prettier --write \"./**/*.ts\" \"./**/*.mjs\"",
     "fix": "concurrently -m 1 -c auto npm:fix:eslint npm:fix:prettier",
@@ -51,6 +52,9 @@
     "shlex": "^2.1.2"
   },
   "devDependencies": {
+    "@types/eslint": "^9.6.0",
+    "@types/eslint__js": "^8.42.3",
+    "@types/eslint-plugin-mocha": "^10.4.0",
     "@types/mocha": "^10.0.7",
     "@types/mock-require": "^2.0.3",
     "@types/node": "^22.1.0",

tsconfig.eslint.json

@@ -0,0 +1,12 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "module": "nodenext",
+    "moduleResolution": "nodenext",
+    "noEmit": true,
+    "allowJs": true
+  },
+  "files": [
+    "eslint.config.mjs"
+  ]
+}