refactor(common): infer flatten helper

Author: kamilmysliwiec

packages/common/decorators/core/dependencies.decorator.ts

@@ -1,13 +1,15 @@
 import { PARAMTYPES_METADATA } from '../../constants';
 
-export function flatten<T extends any[] = any, R extends any[] = any>(
+export function flatten<T extends Array<unknown> = any>(
   arr: T,
-): R {
+): T extends Array<infer R> ? R : never {
   const flat = [].concat(...arr);
-  return (flat.some(Array.isArray) ? flatten(flat) : flat) as R;
+  return flat.some(Array.isArray) ? flatten(flat) : flat;
 }
 
-export const Dependencies = (...dependencies: any[]): ClassDecorator => {
+export const Dependencies = (
+  ...dependencies: Array<unknown>
+): ClassDecorator => {
   const flattenDeps = flatten(dependencies);
   return (target: object) => {
     Reflect.defineMetadata(PARAMTYPES_METADATA, flattenDeps, target);

packages/core/discovery/discovery-service.ts

@@ -1,4 +1,5 @@
 import { flatten, Injectable } from '@nestjs/common';
+import { InstanceWrapper } from '../injector/instance-wrapper';
 import { Module } from '../injector/module';
 import { ModulesContainer } from '../injector/modules-container';
 
@@ -13,15 +14,15 @@ export class DiscoveryService {
   getProviders(
     options: DiscoveryOptions = {},
     modules: Module[] = this.getModules(options),
-  ) {
+  ): InstanceWrapper[] {
     const providers = modules.map(item => [...item.providers.values()]);
     return flatten(providers);
   }
 
   getControllers(
     options: DiscoveryOptions = {},
     modules: Module[] = this.getModules(options),
-  ) {
+  ): InstanceWrapper[] {
     const controllers = modules.map(item => [...item.controllers.values()]);
     return flatten(controllers);
   }