Fix S4158 FP: The analyzer incorrectly concludes the HashSet is guaranteed to be empty

[JelkeH]

Description

HashSet.Remove causes S4158 on an empty HashSet. It should not.

Repro steps

I tried to remove parts of this repro, but I could not make it cleaner without loosing the FP. _ = toDoLater.Remove(item); issues S4158

    public interface IFoo<T>
    {
        public bool someProperty { get; }

        public IEnumerable<T> Successors { get; }
    }

    public static void Func<T>(this ICollection<T> toDo)
        where T : class, IFoo<T>
    {
        Queue<T> toDoNow = new();
        HashSet<T> toDoLater = new();

        foreach (T item in toDo)
        {
            if (item.someProperty)
            {
                toDoNow.Enqueue(item);
            }
            else
            {
                _ = toDoLater.Add(item);
            }
        }

        while (toDoNow.Count > 0)
        {
            T current = toDoNow.Dequeue();

            // Handle current

            foreach (T item in current.Successors)
            {
                _ = toDoLater.Remove(item);
                toDoNow.Enqueue(item);
            }
        }
    }

Expected behavior

S4158 is not issued as the HashSet is not guaranteed to be empty

Actual behavior

S4158 is issued.

Known workarounds

Suppress the issued warning.

Related information

SonarAnalyzer version 9.7.0.75501

[Tim-Pohlmann]

Hi @JelkeH, thanks for the report. I confirm this as an FP and created a PR to fix it. You can follow the progress here:
#8066