I’m trying to write jest unit tests for a basic angular component. But I seem to have stumbled on a strange error. A minimal repro is hosted here.

When running the test using VS Code and the Jest Runner extension, I get the following error

TypeError: Cannot read properties of undefined (reading ‘Symbol(SIGNAL)’)

Here’s my specfile

@Component({
    selector: 'demo-checkbox-test',
    template: `
        <div bsCheckboxGroup>
            <bs-checkbox>
                This is a checkbox
            </bs-checkbox>
        </div>`
})
class BsCheckboxTestComponent { }

describe('BsCheckboxComponent', () => {
    let component: BsCheckboxTestComponent;
    let fixture: ComponentFixture<BsCheckboxTestComponent>;

    beforeEach(async () => {
        await TestBed.configureTestingModule({
            imports: [
                // Mock dependencies
            ],
            declarations: [
                // Unit to test
                BsCheckboxComponent,

                // Mock dependencies
                MockDirective(BsCheckboxGroupDirective),

                // Testbench
                BsCheckboxTestComponent,
            ]
        }).compileComponents();

        fixture = TestBed.createComponent(BsCheckboxTestComponent);
        await fixture.whenStable();
        component = fixture.componentInstance;
        fixture.detectChanges();
    });

    it('should create', () => {
        expect(component).toBeTruthy();
    });
});

And here’s my unit-to-test

@Component({
    selector: 'bs-checkbox',
    template: `
        <label>
            <input type="checkbox">
        </label>`
})
export class BsCheckboxComponent {}

The testbed mocks the following directive:

@Directive({
    selector: '[bsCheckboxGroup]',
})
export class BsCheckboxGroupDirective {
    name = input.required<string>();
    children = contentChildren(forwardRef(() => BsCheckboxComponent));
    // @ContentChildren(forwardRef(() => BsCheckboxComponent)) children!: QueryList<BsCheckboxComponent>;
}

Remarkably when using the @ContentChildren(...) decorator instead of the contentChildren(...) function, the test runs perfectly fine.

What’s causing this? How can I fix it?