JustDummies

Any.Reproducibly takes a synchronous Action. An async lambda bound to it becomes async void: the body runs to its first await, then its continuation — every assertion after that await — runs after the call has already returned, and the exception escapes the reproducible scope entirely. The test passes green even though the body failed.

Pass the asynchronous body to Any.ReproduciblyAsync(Func<Task>) and await it. For a synchronous body, keep Any.Reproducibly(() => { ... }).

Noncompliant

[Fact]
public void A_20_percent_discount_takes_a_fifth_off_the_order() {
    Any.Reproducibly(async () => {          // JD001: the async body runs as 'async void'
        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);     // this failure never reaches the test runner
    });
}

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);
    });
}