[consistent-type-imports] conflict with importsNotUsedAsValues compile option.

[CatsMiaow]

• 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/consistent-type-imports": "error"
  }
}

tsconfig.json

{
  "compilerOptions": {
    "importsNotUsedAsValues": "error",
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true
  }
}

import type { Request, Response } from 'express';

@Get()
public hello(@Req() req: Request, @Res() res: Response): void {}

Expected Result

Actual Result

In the code, Request and Response are used only as type information.

Additional Info

// This code also causes the same problem.
import type { Redis } from 'ioredis';

class Test {
  constructor(@Inject(REDIS) private redis: Redis) {}
}

Versions

package	version
@typescript-eslint/eslint-plugin	5.6.0
@typescript-eslint/parser	5.6.0
TypeScript	4.5.2
ESLint	8.3.0
node	16.13.0

[bradzacher]

TS is technically correct here - but it's inconsistent in its behaviour and impossible for us to properly reproduce in the lint rule.

emitDecoratorMetadata implicitly uses values - for example if you import a class and use it, TS will emit code which uses the class at runtime, even though your code as written only uses it as a type. This is consistent for most things that can be assigned to a type annotation - TS will implicitly use the value at runtime.

However if you pass an actual type (an interface, for example), TS will not use it as a value, and will instead use it as a type - it will use the type to inform the codegen. For example an interface will cause TS to emit Object, but a function type will cause TS to emit Function. TS does this across module boundaries (violating its own isolated modules build restrictions).

This is one of the few, rare cases where the two will disagree. Unfortunately because you need type information to do this, we cannot exactly replicate this behaviour in our lint rule. See other discussion on #3108.

You should either use the TS option, or our lint rule - not both.