Assert EqualTo().Within().Seconds does not work with DateTimes in NUnit 3.13

[smkanadl]

I have several Asserts in the following style comparing DateTimes to be equal within small amount of time. This worked well in NUnit 3.12.
Assert.That(DateTime.UtcNow - TimeSpan.FromSeconds(5), Is.EqualTo(DateTime.UtcNow).Within(1).Seconds);

Message: 
      Expected: 2021-01-15 11:44:01.8274509 +/- 00:00:01
      But was:  2021-01-15 11:43:56.8274456

Updating to 3.13 is OK when the test passes, but if the test breaks (for whatever reason), I get the following message.

Message: 
    System.ArgumentException : Both arguments must be numeric

Obviously, NUnit struggles to create the actual message.

Stacktrace:

Numerics.Difference(Object expected, Object actual, Boolean isAbsolute) line 447
Numerics.Difference(Object expected, Object actual, ToleranceMode toleranceMode) line 436
TextMessageWriter.WriteDifferenceLine(Object expected, Object actual, Tolerance tolerance) line 330
TextMessageWriter.DisplayDifferences(Object expected, Object actual, Tolerance tolerance) line 181
EqualConstraintResult.DisplayDifferences(MessageWriter writer, Object expected, Object actual, Int32 depth) line 96
EqualConstraintResult.WriteMessageTo(MessageWriter writer) line 82
Assert.ReportFailure(ConstraintResult result, String message, Object[] args) line 381
Assert.That[TActual](TActual actual, IResolveConstraint expression, String message, Object[] args) line 229
Assert.That[TActual](TActual actual, IResolveConstraint expression) line 210

[mikkelbu]

I think this is an unfortunate side-effect of #3697. I've placed this report in the milestone that we aim to release in a couple of weeks.

[smkanadl]

Many thanks for taking care!

[Antash]

@mikkelbu that's a shame :|
Will fix shortly

[mikkelbu]

Thanks @Antash 👍. I'll assign the issue to you, please write if you have any questions.

[Antash]

@mikkelbu fixed. I'm lucky to face CI issues again though :)

[manfred-brands]

I'm facing the same issue, but not with datetime, but with a custom type. Is there a beta feed for nuget I can test before release?

[mikkelbu]

@manfred-brands The prerelease feed is here - https://www.myget.org/feed/nunit/package/nuget/NUnit. But looking at the PR I don't think it will help you, as it extends the handling, so that we also handle DateTimes as well as numerics.

Either we should not fail for "unknown" cases or we should handle then generically. Can you provide a small example that reproduces the problem (probably in a new issue).

[manfred-brands]

@mikkelbu In my case we had a type implementing IEquatable<T>. Looking at the code, the EquatablesComparer ignores the Tolerance completely.

Should we add a rule to nunit.analyzer to complain that .Within is not used and should be removed for non-numeric comparisons?

I added #3736 where the error message also throws for Tuples.