[Fact]
public void When_injecting_a_EqualityComparer_to_ContainEquivalentOf_it_should_be_used()
{
    var allItems = Enumerable.Range(0, 10).Select(x => new CustomClass(x)).ToList();
    var subset = Enumerable.Range(3, 3).Select(x => new CustomClass(x)).ToList();

    // This works.
    var matchingItems = allItems.Intersect(subset, new SimpleCustomClassComparer()).ToList();
    matchingItems.Select(x => x.Id).Should().BeEquivalentTo(new[] { 3, 4, 5 });

    // This doesn't work because the EqualityComparer isn't called by FluentAssertions.
    allItems.Should().ContainEquivalentOf(subset, opt => opt.Using(new SimpleCustomClassComparer()));
}

private class CustomClass
{
    public CustomClass(int id)
    {
        this.Id = id;
    }

    public int Id { get; set; }

    public Guid Guid => Guid.NewGuid();
}

private class SimpleCustomClassComparer : IEqualityComparer<CustomClass>
{
    public bool Equals(CustomClass x, CustomClass y)
    {
        return x?.Id == y?.Id;
    }

    public int GetHashCode(CustomClass obj)
    {
        return obj.Id;
    }
}

allItems.Should().ContainEquivalentOf(subset, opt => opt.Using(new SimpleCustomClassComparer()));

// Workaround: Eliminate the need for a custom comparer
var allItemsIds = allItems.Select(x => x.Id);
var subsetIds = subset.Select(x => x.Id);

allItemsIds.Should().Contain(subsetIds);