Nest dependency injection can't resolve type-only imports

[vegerot]

See typescript-eslint/typescript-eslint#2559 for discussion there

Bug Report

Since Typescript 3.8, Typescript has supported type-only imports and exports. This feature is incompatible with constructor dependency injection.

Current behavior

If you do a type-only import, you get an error like

Error: Nest can't resolve dependencies of the AppController (?, Function, Function, Function, UserSessionRepository). Please make sure that the argument Function at index [0] is available in the RootTestModule context.

Input Code

import {
  Controller,
} from '@nestjs/common';
import type { Logger } from '@nestjs/common';
import type { ConfigService } from './config/config.service';
import { UserSessionRepository } from '../entities/user-session/user.session.repository';
import type { UsersService } from './users/users.service';
import type { AuthService } from './auth/auth.service';

@Controller()
export class AppController {
  private readonly LANDING_URL: string;
  private readonly ROUTE: string;
  private readonly OAUTH_GATEWAY: string;

  constructor(
    private readonly config: ConfigService,
    private readonly usersService: UsersService,
    private readonly logger: Logger,
    private readonly authService: AuthService,
    @InjectRepository(UserSessionRepository)
    private readonly userSessionRepository: UserSessionRepository,
  ) {
    this.LANDING_URL = config.get('LANDING_URL');
    this.ROUTE = config.get('ROUTE');
    this.OAUTH_GATEWAY = config.get('OAUTH_GATEWAY');

Expected behavior

The dependencies should be injected as per usual.

Possible Solution

nest.js should be able to resolve type-only imports. Possibly by doing its injection earlier in the typescript build phase

Environment

Nest version: 7.4.4

Others:
Please see https://github.com/typescript-eslint/typescript-eslint/issues/2559 for discussion there too

[jmcdo29]

The dependency injection runs during the use of node, not of tsc. This means that what's happening during the build phase is simply just transpiling Typescript into JavaScript, during this time, typed imports are checked for proper types, and then become {} just like interfaces. This isn't something that Nest will be able to get around, but rather something that should be taken into account during development. If you want to use class interfaces, or injection tokens with custom providers, that's something that can be managed, but Nest's DI system cannot get around what Typescript does with import type.