[@typescript-eslint/no-unused-vars] False positive for an interface used as a generic parameter on a function with a decorator.

[timecatalyst]

Sorry, I know a lot of similar issues like this have been reported, but none of them appear to address my specific problem. I'm working in a fresh NestJS project (generated using @nestjs/cli) and for some reason the @Get() decorator is causing the linter to flag an interface as being unused.

I had originally reported the issue on the NestJS repo, but they suggested it may be a linter problem and to ask about it here (See my comment near the bottom: nestjs/nest#4684). I accept the possibility that I might just have something wrong in my configuration, but I can't for the life of me figure out what that may be.

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have read the FAQ and my problem is not listed.

Repro

{
  "rules": {
    "@typescript-eslint/interface-name-prefix": "off",
    "@typescript-eslint/explicit-function-return-type": "off",
    "@typescript-eslint/explicit-module-boundary-types": "off",
    "@typescript-eslint/no-explicit-any": "off",
  },
};

As I mentioned earlier, this is a brand new project generated freshly from the NestJS CLI. The only upgrade the @typescript-eslint libraries to the latest versions.

The linter generates a warning on the interface Foo at line 3, despite its use at line 10.

If I remove the @Get() decorator, or rewrite doSomething() to not use a Promise<>, the warning goes away.

 1 import { Controller, Get } from '@nestjs/common';
 2
 3 interface Foo {
 4   foo: string;
 5 }
 6
 7 @Controller('foo') 
 8 export class FooController {
 9   @Get()
10   async doSomething(): Promise<Foo> {
11     return Promise.resolve({ foo: 'bar' });
12   }
13 }

tsconfig.json:

{
  "compilerOptions": {
    "module": "commonjs",
    "declaration": true,
    "removeComments": true,
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "allowSyntheticDefaultImports": true,
    "target": "es2017",
    "sourceMap": true,
    "outDir": "./dist",
    "baseUrl": "./",
    "incremental": true
  }
}

Expected Result

No warning should occur.

Actual Result

ESLint: 'Foo' is defined but never used.(@typescript-eslint/no-unused-vars)

Additional Info

None

Versions

package	version
@typescript-eslint/eslint-plugin	4.14.0
@typescript-eslint/parser	4.14.0
TypeScript	4.0.5
ESLint	7.12.1
node	12.8.3

[bradzacher]

Same as #2941, #2942, #2945, #2947, #2950, #2957

[timecatalyst]

Sorry, but I'm still seeing the same problem after upgrading my libraries to v4.14.1. Same reproduction steps as before, a class method decorator combined with a generic return type. Here's a more minimalistic version that removes NestJS from the equation:

 1 interface Foo {
 2   foo: string;
 3 }
 4
 5 function MyDecorator() {
 6   return (target, propertyKey: string, descriptor: PropertyDescriptor) =>
 7     console.log('MyDecorator(): called');
 8 }
 9
10 export class FooController {
11   @MyDecorator()
12   async doSomething(): Promise<Foo> {
13     return Promise.resolve({ foo: 'foo' });
14   }
15 }

[bradzacher]

Nuke your node modules and reinstall.
Also restart your IDE.

This is fixed (I just tested your code against master)

[timecatalyst]

Yeah, I've nuked node modules and restarted my IDE several times to no avail. I suppose it's possible something might be conflicting with it, but I'm not sure what that would be at this point. Here are my deps:

 "dependencies": {
    "@nestjs/common": "^7.5.1",
    "@nestjs/core": "^7.5.1",
    "@nestjs/platform-express": "^7.5.1",
    "reflect-metadata": "^0.1.13",
    "rimraf": "^3.0.2",
    "rxjs": "^6.6.3"
  },
  "devDependencies": {
    "@nestjs/cli": "^7.5.1",
    "@nestjs/schematics": "^7.1.3",
    "@nestjs/testing": "^7.5.1",
    "@types/express": "^4.17.8",
    "@types/jest": "^26.0.15",
    "@types/node": "^14.14.6",
    "@types/supertest": "^2.0.10",
    "@typescript-eslint/eslint-plugin": "^4.14.1",
    "@typescript-eslint/parser": "^4.14.1",
    "eslint": "^7.18.0",
    "eslint-config-prettier": "7.1.0",
    "eslint-plugin-jsx-a11y": "^6.4.1",
    "eslint-plugin-prettier": "^3.1.4",
    "jest": "^26.6.3",
    "prettier": "^2.1.2",
    "supertest": "^6.0.0",
    "ts-jest": "^26.4.3",
    "ts-loader": "^8.0.14",
    "ts-node": "^9.1.1",
    "tsconfig-paths": "^3.9.0",
    "typescript": "^4.1.3"
  },

[bradzacher]

There may be multiple versions installed?
https://github.com/typescript-eslint/typescript-eslint/blob/master/docs/getting-started/linting/FAQ.md#how-do-i-check-to-see-what-versions-are-installed

Hard to say exactly - it all depends on your project setup.

[timecatalyst]

Looks like I'm on the right version. I'm going to try to build the project up from scratch and see where it gets me. I'll post back if I find anything. Thanks for the feedback.

% yarn list @typescript-eslint/eslint-plugin @typescript-eslint/parser
yarn list v1.22.10
warning Filtering by arguments is deprecated. Please use the pattern option instead.
├─ @typescript-eslint/eslint-plugin@4.14.1
└─ @typescript-eslint/parser@4.14.1
✨  Done in 0.33s.

[timecatalyst]

Okay, I rebuild the project with just typescript and linting:

package.json:
{
  "name": "eslint-test",
  "version": "0.0.1",
  "description": "",
  "author": "",
  "private": true,
  "license": "UNLICENSED",
  "scripts": {},
  "dependencies": {},
  "devDependencies": {
    "@types/node": "^14.14.22",
    "@typescript-eslint/eslint-plugin": "^4.14.1",
    "@typescript-eslint/parser": "^4.14.1",
    "eslint": "^7.18.0",
    "eslint-config-prettier": "^7.2.0",
    "eslint-plugin-prettier": "^3.3.1",
    "prettier": "^2.2.1",
    "ts-loader": "^8.0.14",
    "ts-node": "^9.1.1",
    "tsconfig-paths": "^3.9.0",
    "typescript": "^4.1.3"
  }
}

tsconfig.json:
{
  "compilerOptions": {
    "module": "commonjs",
    "declaration": true,
    "removeComments": true,
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "allowSyntheticDefaultImports": true,
    "target": "es2017",
    "sourceMap": true,
    "outDir": "./dist",
    "baseUrl": "./",
    "incremental": true
  }
}

I have a single index.ts containing the same test code from before.

Using only the above files, I did not receive a warning on interface Foo. But, when I added in the .eslintrc.js file from the NestJS repo, the warning reappeared. I did some digging and found that removing plugin:@typescript-eslint/recommended from the extends array makes the warning disappear (along with other legitimate no-unused-vars warnings). I suspect there may be some invalid usage going on. Here's the eslintrc:

module.exports = {
  parser: '@typescript-eslint/parser',
  parserOptions: {
    project: 'tsconfig.json',
    sourceType: 'module',
  },
  plugins: ['@typescript-eslint/eslint-plugin'],
  extends: [
    'plugin:@typescript-eslint/eslint-recommended',
    'plugin:@typescript-eslint/recommended',
    'plugin:@typescript-eslint/recommended-requiring-type-checking',
    'prettier/@typescript-eslint',
    'plugin:prettier/recommended',
  ],
  root: true,
  env: {
    node: true,
    jest: true,
  },
  ignorePatterns: ['.eslintrc.js'],
  rules: {
    '@typescript-eslint/interface-name-prefix': 'off',
    '@typescript-eslint/explicit-function-return-type': 'off',
    '@typescript-eslint/explicit-module-boundary-types': 'off',
    '@typescript-eslint/no-explicit-any': 'off',
  },
};

[timecatalyst]

@bradzacher I think I found the culprit: "emitDecoratorMetadata": true, in tsconfig.json. If I remove that key, or set the value to false, the linter error disappears.

[bradzacher]

See #2972