Bug: Injection of mocked service fails for nested service ... but only sometimes ...

[ptandler]

Description of the bug

We started using the very nice ng-mocks for our test, but I have trouble getting the mock injection right for this case:

• OurService that uses the DatetimeService and NumberFormatterService.
• NumberFormatterService also uses the DatetimeService.
• DatetimeService should be mocked.

I also posted this issue on StackOverflow, but after realizing, that this problem happens only sometimes and quite randomly, I could imagine that this is a bug, so I decided to open an issue here. In fact, it could be a timing issue. (Or caching?)

An example of the bug

describe('OurService', () => {
  MockInstance.scope();

  let service: OurService;

  beforeEach(async () => {
    MockInstance(DatetimeService, 'locale', () => 'en-US');

    await MockBuilder(OurService)
      .mock(DatetimeService)
      .provide(NumberFormatterService);

    service = TestBed.inject(OurService);
  });

  it('should be created', () => {
    expect(service).toBeTruthy();
  });

  test('test setup is correct', () => {
    expect(TestBed.inject(DatetimeService).locale()).toEqual('en-US'); // works
    expect(service.datetimeService.locale()).toEqual('en-US'); // works
    expect(service.numberFormatter.datetimeService.locale()).toEqual('en-US'); // <== this fails (sometimes): locale is undefined
  });
  
  // ...
});

Expected vs actual behavior

I would expect that the mocked Instance on DatetimeService is also always injected into the NumberFormatter.