JustDummies

Any.ReproduciblyAsync returns a Task that faults with the body’s exception. Discarding it — as a standalone statement, or via _ = — lets a failing test pass green, because the failure is never observed. await the returned task.

The compiler’s own CS4014 does not catch this in a synchronous (void) test method, which is exactly where the mistake is easiest to make.

Noncompliant

[Fact]
public void A_20_percent_discount_takes_a_fifth_off_the_order() {
    Any.ReproduciblyAsync(async () => {     // JD002: the returned task is discarded
        string anyReference = Any.String().StartingWith("ORD-").WithLength(12).Generate();
        string anyCustomer  = Any.String().Alpha().WithLengthBetween(1, 50).Generate();

        Order order = new Order(anyReference, anyCustomer, amount: 100m);
        order.ApplyDiscount(20);

        await _repository.SaveAsync(order);

        Assert.Equal(80m, order.Total);
    });
}

_ = Any.ReproduciblyAsync(async () => { /* ... */ }); // JD002: the returned task is discarded

Compliant

[Fact]
public async Task A_20_percent_discount_takes_a_fifth_off_the_order() {
    await Any.ReproduciblyAsync(async () => {
        string anyReference = Any.String().StartingWith("ORD-").WithLength(12).Generate();
        string anyCustomer  = Any.String().Alpha().WithLengthBetween(1, 50).Generate();

        Order order = new Order(anyReference, anyCustomer, amount: 100m);
        order.ApplyDiscount(20);

        await _repository.SaveAsync(order);

        Assert.Equal(80m, order.Total);
    });
}