set.ExceptWith(set) leaves every other element in ContiguousSet and throws in OrderedSet/InsertionOrderSet

[matt-edmondson]

What's wrong

ExceptWith in all three sets loops foreach (T item in other) Remove(item);. When other is the set itself, the set is modified while it is being enumerated:

• Containers/ContiguousSet.cs (~line 370): the hand-written enumerator has no version check, and each Remove shifts the array, so every other element is skipped.
• Containers/OrderedSet.cs (~line 380) and Containers/InsertionOrderSet.cs (~line 299) enumerate the underlying List<T>, so they throw.

var s = new ContiguousSet<int> { 1, 2, 3, 4 };
s.ExceptWith(s);                         // Count=2, [2, 4]

var o = new OrderedSet<int> { 1, 2, 3, 4 };
o.ExceptWith(o);                         // InvalidOperationException: Collection was modified

var i = new InsertionOrderSet<int> { 1, 2, 3, 4 };
i.ExceptWith(i);                         // InvalidOperationException

var h = new HashSet<int> { 1, 2, 3, 4 };
h.ExceptWith(h);                         // [] (BCL reference behaviour)

Reproduced against the current main build.

Why it matters

These types implement ISet<T>, and HashSet<T>/SortedSet<T> define x.ExceptWith(x) as clearing the set. Code written against ISet<T> (for example "remove everything in selection from items" where both happen to be the same instance) either crashes or silently leaves half the elements behind, depending on which set type it was given.

Suggested fix

At the top of each ExceptWith, after the null check:

if (ReferenceEquals(other, this))
{
    Clear();
    return;
}

The other set operations already copy other into a temporary set first, so they are not affected.

Acceptance criteria

• x.ExceptWith(x) leaves x empty for ContiguousSet, OrderedSet and InsertionOrderSet.
• A regression test per type.