Honor OrderedSet's own comparer in the set operations

[matt-edmondson]

Fixes #51

The problem

OrderedSet<T> decides membership two different ways. Add, Remove and Contains route through BinarySearch, which uses the IComparer<T> the set was constructed with. But six of the ISet<T> operations materialized the other collection into a HashSet<T> built with the default constructor, so both membership tests and cardinality were answered by EqualityComparer<T>.Default instead:

• IntersectWith
• SymmetricExceptWith
• IsSubsetOf
• IsProperSubsetOf
• IsProperSupersetOf
• SetEquals

For any set built with a comparer that defines a different equality notion than the default, the two disagree:

OrderedSet<string> set = new(StringComparer.OrdinalIgnoreCase);
set.Add("Hello");
set.Contains("HELLO");        // true — respects the custom comparer
set.IntersectWith(["HELLO"]);
set.Contains("Hello");        // false — the item was wrongly evicted

The fix

Build the temporary set with the set's own comparer, via a small private ToComparerSet helper, so membership and distinct-element counts are decided by the same rule as Contains. This mirrors the sibling ContiguousSet<T>.IntersectWith, which already threads its comparer into the temporary set.

A HashSet<T> cannot be used here: an IEqualityComparer<T> adapter over an IComparer<T> has no way to produce hash codes consistent with that comparer's equality. Building an OrderedSet<T> with the same comparer sidesteps that and keeps membership at O(log n).

IsProperSubsetOf and SetEquals now test membership directly rather than calling back into IsSubsetOf, which would have rebuilt the same temporary set a second time.

Nothing changes for a set using the default comparer, and no public API changed.

Tests

Seven tests added to OrderedSetTests.cs, all using StringComparer.OrdinalIgnoreCase so the set's comparer and default equality disagree. They cover every affected operation, including the cases where the bug showed up in the count rather than the membership test (SetEquals(["HELLO", "hello", "WORLD"]) on a two-element set).

• Baseline on main: 357 passed.
• With this change: 364 passed, 0 failed.
• Reverting only OrderedSet.cs and re-running: 6 of the 7 new tests fail. The seventh (IsProperSubsetOf_CustomComparer_CountsDistinctElementsUnderThatComparer) asserts a false the old code also happened to return, and is kept as a regression guard.

dotnet build Containers.sln is clean — 0 warnings, 0 errors.

🤖 Generated with Claude Code

https://claude.ai/code/session_01K19wdK61qatQNZ7MvvVgvB

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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