test(core): add multiple providers resolution integration tests

integration/injector/e2e/multiple-providers.spec.ts

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+import { Test } from '@nestjs/testing';
+import { expect } from 'chai';
+import { MultipleProvidersModule } from '../src/multiple-providers/multiple-providers.module';
+
+describe('Multiple providers under the same token ("each" feature)', () => {
+  describe('get()', () => {
+    it('should return an array of providers', async () => {
+      const builder = Test.createTestingModule({
+        imports: [MultipleProvidersModule],
+      });
+      const testingModule = await builder.compile();
+
+      const multiProviderInstances = testingModule.get<string>(
+        'MULTI_PROVIDER',
+        {
+          each: true,
+        },
+      );
+
+      // make sure "multiProviderInstances" is string[] not string
+      // @ts-expect-error
+      multiProviderInstances.charAt;
+
+      expect(multiProviderInstances).to.be.eql(['A', 'B', 'C']);
+    });
+  });
+  describe('resolve()', () => {
+    it('should return an array of providers', async () => {
+      const builder = Test.createTestingModule({
+        imports: [MultipleProvidersModule],
+      });
+      const testingModule = await builder.compile();
+
+      const multiProviderInstances = await testingModule.resolve<string>(
+        'REQ_SCOPED_MULTI_PROVIDER',
+        undefined,
+        {
+          each: true,
+        },
+      );
+
+      // make sure "multiProviderInstances" is string[] not string
+      // @ts-expect-error
+      multiProviderInstances.charAt;
+
+      expect(multiProviderInstances).to.be.eql(['A', 'B', 'C']);
+    });
+  });
+});

integration/injector/src/multiple-providers/a.module.ts

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+import { Module, Scope } from '@nestjs/common';
+
+@Module({
+  providers: [
+    {
+      provide: 'MULTI_PROVIDER',
+      useValue: 'A',
+    },
+    {
+      provide: 'REQ_SCOPED_MULTI_PROVIDER',
+      useFactory: () => 'A',
+      scope: Scope.REQUEST,
+    },
+  ],
+})
+export class AModule {}

integration/injector/src/multiple-providers/b.module.ts

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+import { Module, Scope } from '@nestjs/common';
+
+@Module({
+  providers: [
+    {
+      provide: 'MULTI_PROVIDER',
+      useValue: 'B',
+    },
+    {
+      provide: 'REQ_SCOPED_MULTI_PROVIDER',
+      useFactory: () => 'B',
+      scope: Scope.REQUEST,
+    },
+  ],
+})
+export class BModule {}

integration/injector/src/multiple-providers/c.module.ts

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+import { Module, Scope } from '@nestjs/common';
+
+@Module({
+  providers: [
+    {
+      provide: 'MULTI_PROVIDER',
+      useValue: 'C',
+    },
+    {
+      provide: 'REQ_SCOPED_MULTI_PROVIDER',
+      useFactory: () => 'C',
+      scope: Scope.REQUEST,
+    },
+  ],
+})
+export class CModule {}

integration/injector/src/multiple-providers/multiple-providers.module.ts

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+import { Module } from '@nestjs/common';
+import { AModule } from './a.module';
+import { BModule } from './b.module';
+import { CModule } from './c.module';
+
+@Module({
+  imports: [AModule, BModule, CModule],
+})
+export class MultipleProvidersModule {}