Is.Not.EqualTo does not work correctly with Using comparer

[igor-vyatkin]

If we apply custom comparer with Using to Is.Not.EqualTo then Not is ignored and it starts to work as Is.EqualTo with Using.

Test below fails but should pass:

[Test]
public void UsesProvidedIComparerWithNot()
{
    var comparer = new ObjectComparer();
    Assert.Multiple(() =>
    {
        Assert.That(2 + 3, Is.Not.EqualTo(4).Using(comparer));
        Assert.That(comparer.WasCalled, "Comparer was not called");
    });
}

It produces the followong error:

Assert.That(2 + 3, Is.Not.EqualTo(4).Using(comparer))
  Expected: 4
  But was:  5

If just remove .Using(comparer) it starts to work as expected.

Tested with latest NUnit 4.3.2 and .NET 8.

[stevenaw]

Thanks for the report @IgorVyatkin
I think I can confirm based on a repro I've modified from your example and pushed to https://github.com/nunit/nunit.issues/blob/main/Issue4964/UnitTest1.cs

However, can you please show a sample implementation of your ObjectComparer ? It can be very simplified, I'm primarily curious which interfaces it implements to confirm if your code ends up calling a different Using() overload than my own.

[igor-vyatkin]

@stevenaw I used NUnit tests to prepare an exapmle, so ObjectComparer is taken from https://github.com/nunit/nunit/blob/main/src/NUnitFramework/tests/TestUtilities/Comparers/ObjectComparer.cs

Locally I got this problem with IEqualityComparer<string?> comparer implementation.

[manfred-brands]

@stevenaw The problem is with all comparers. I know why and will put up a PR hopefully today or in the weekend.