Hint to replace HashSet<T> with ISet<T>, not ICollection<T>

Given this method:

    private static void Add<TValue>(HashSet<TValue> values) {
      values.Add(default(TValue));
      if(values.Contains(default(TValue))) {
        values.Add(default(TValue));
      }
    }

Resharper offers a hint (suggestion?) to replace HashSet<TValue> with ISet<TValue>. It could have suggested ICollection<TValue> (ISet<T>'s base interface) since Add() only uses collection methods. Not sure this is actually a bug. The ISet<T> signature *is* more clear, and if the method relies on the collection having certain properties (e.g. no duplicates, fast lookup), then the caller *should* only call it with ISet<T> instances.

Anyway, I thought I'd point it out since the behavior is somewhat unexpected.