Instances that are mocked cannot be asserted when they are used as parameters to mocked methods

[joeythomaschaske]

Given these classes

public class CarFactory {
    public Car getCustomizedCar() {
        Car car = Car.newInstance('Mercedes');
        CarCustomizer customizer = CarCustomizer.newInstance();
        customizer.customize(car);
        car.save();
        return car;
    }
}

public class Car {
    @TestVisible
    private static Car instance;

    private String type;

    public static Car newInstance(String type) {
        if (instance == null) {
            return new Car(type);
        }

        return instance;
    }

    public Car(String type) {
        this.type = type;
    }

    public void save() {
        
    }
}

public class CarCustomizer {
    @TestVisible
    private static CarCustomizer instance;

    public static CarCustomizer newInstance() {
        if (instance == null) {
            return new CarCustomizer();
        }

        return instance;
    }

    public Car customize(Car car) {
        // customize
        return car;
    }
}

And this test class

@IsTest
private class CarFactoryTest {
    @IsTest
    private static void customizesTheCar() {
        // given
        MockClass carMock = new MockClass();
        MockMethod saveMock = new MockMethod('save', new List<Object> { null });
        carMock.addMockMethod(saveMock);

        MockClass carCustomizerMock = new MockClass();
        MockMethod customizeMock = new MockMethod('customize', new List<Object> { null });
        carCustomizerMock.addMockMethod(customizeMock);

        Car.instance = (Car) Test.createStub(Car.class, carMock);
        CarCustomizer.instance = (CarCustomizer) Test.createStub(CarCustomizer.class, carCustomizerMock);

        // when
        CarFactory.getCustomizedCar();

        // then
        Assert.areEqual(1, customizeMock.getTimesCalled());

        Car expectedCar = new Car('Ford');
        Assert.areNotEqual(expectedCar, customizeMock.getNthCalledWith(1)[0]);
    }
}

This test won't work as expected because Car is being mocked with a constant type. We won't be able to assert that the correct type of car is being passed to customize, in this case Mercedes.

It would be useful to include a mechanism to allow real instances to be created instead of always returning the mock