Is.EqualTo with empty ValueTuple throws System.NotSupportedException "Specified Tolerance not supported for instances of type 'System.ValueTuple' and 'System.ValueTuple'" after updating to NUnit 4.2.2

[KrzysFR]

After updating from NUnit 4.1 to 4.2.2, I have the following error when comparing the value of a ValueTuple (empty tuple) with Is.EqualTo constraint:

System.NotSupportedException : Specified Tolerance not supported for instances of type 'System.ValueTuple' and 'System.ValueTuple'

The issue does NOT happen when comparing to a non-empty tuple, or with the Is.Not.EqualTo constraint.

Sample code:

[TestFixture]
public class ReproTest
{
	[Test]
	public void Test_ValueTuple_Equality()
	{
		Assert.Multiple(() =>
		{
			// 4.1: PASS, 4.2.2: **FAIL**
			Assert.That(ValueTuple.Create(), Is.EqualTo(ValueTuple.Create()), "() == ()"); // => throws NotSupportedException !

			// 4.1: PASS, 4.2.2: PASS
			Assert.That(ValueTuple.Create(123), Is.EqualTo(ValueTuple.Create(123)), "(123,) == (123,)");

			// 4.1: PASS, 4.2.2: PASS
			Assert.That(ValueTuple.Create(123), Is.Not.EqualTo(ValueTuple.Create()), "(123,) != ()");

			// 4.1: PASS, 4.2.2: PASS
			Assert.That(ValueTuple.Create(), Is.Not.EqualTo(ValueTuple.Create(123)), "() != (123,)");
		});
	}
}

Observed error in 4.2.2:

System.NotSupportedException : Specified Tolerance not supported for instances of type 'System.ValueTuple' and 'System.ValueTuple'
   at NUnit.Framework.Constraints.NUnitEqualityComparer.AreEqual(Object x, Object y, Tolerance& tolerance)
   at NUnit.Framework.Constraints.EqualConstraint.ApplyTo[TActual](TActual actual)
   at NUnit.Framework.Assert.That[TActual](TActual actual, IResolveConstraint expression, NUnitString message, String actualExpression, String constraintExpression)
   at ReproTest.Test_ValueTuple_Equality()
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)

[KrzysFR]

Looking at the code, I think this can be caused by this test:

nunit/src/NUnitFramework/framework/Constraints/Comparers/TupleComparerBase.cs

Lines 42 to 43 in 116802d

	if (redoWithoutTolerance == (1U << numberOfGenericArgs) - 1)
	return EqualMethodResult.ToleranceNotSupported;

Here, both redoWithoutTolerance and numberOfGenericArgs should be equal to 0 for ValueTuple, and (1U << numberOfGenericArgs) - 1 will return 0.

[stevenaw]

Thanks for the report @KrzysFR ! I'm not at a computer myself to confirm the behavior you're seeing, but your diagnosis there looks sound. Would you be interested in contributing a fix?

[manfred-brands]

I didn't think of Tuples without any arguments.

[KrzysFR]

I've had the same issue multiple times, since ValueTuple is not the same as ValueTuple<...> (same issue with Task/Task<T>) !

I don't have an easy way to rebuild/test the NUnit project right now, but I think the fix could be as simple as adding the following line, but I'm not sure if this should be done before or after the call to state.PushComparison

 if (numberOfGenericArgs == 0) 
     return EqualMethodResult.ComparedEqual;

[manfred-brands]

Ok, I'll fix it.

[manfred-brands]

The API is a bit unexpected and inconsistent.
To me a (Value)Tuple with 1 argument doesn't make any sense, let alone a ValueTuple with no arguments.
Note that there is no Tuple.Create with no arguments, but ValueTuple has one.

[KrzysFR]

Tuples with 0 or 1 argument are more likely to be seen when dealing with tuples that behave like vectors, for example via the System.Runtime.CompilerServices.ITuple interface, where a tuple can have any length (0, 1, 2, etc...), and support operations such as Append, Concat, Tail, Slice, ...

The empty tuple would be used as the initial value (similar to en empty list), or as the marker of "no more items" when popping values from a vector until it is "done".