Fix IsEqualTo assertion for double.NaN values

[Copilot]

Problem

The IsEqualTo assertion was failing when comparing double.NaN values, even though both values were NaN. This is a regression compared to v0.67.19.

[Test]
public async Task Test1()
{
    await Assert.That(double.NaN).IsEqualTo(double.NaN);
}

Result:

AssertionException: Expected to be NaN
but found NaN

Interestingly, float.NaN comparisons worked correctly, but double.NaN did not.

Root Cause

The DoubleEqualsAssertion class was using the == operator to compare double values:

if (value == _expected)
{
    return Task.FromResult(AssertionResult.Passed);
}

According to the IEEE 754 floating-point standard, NaN == NaN always returns false, causing the assertion to fail incorrectly.

Solution

Changed the comparison to use double.Equals() instead:

if (double.Equals(value, _expected))
{
    return Task.FromResult(AssertionResult.Passed);
}

The double.Equals() method correctly treats NaN values as equal, which is the expected behavior for equality assertions.

Why float.NaN Worked

float.NaN comparisons worked correctly because there's no specialized FloatEqualsAssertion. Instead, float values use the generic EqualsAssertion<float> which internally uses EqualityComparer<float>.Default.Equals(), which already handles NaN correctly.

Tests Added

Added comprehensive tests for special floating-point values:

• Double_NaN_EqualsTo_NaN_Success()
• Double_PositiveInfinity_EqualsTo_PositiveInfinity_Success()
• Double_NegativeInfinity_EqualsTo_NegativeInfinity_Success()

All tests pass successfully.

Fixes #[issue_number]

Original prompt

---

This section details on the original issue you should resolve

<issue_title>IsEqualTo does not handle double.NaN correctly</issue_title>
<issue_description>```cs
[Test]
public async Task Test1()
{
await Assert.That(double.NaN).IsEqualTo(double.NaN);
}

Result:

AssertionException: Expected to be NaN
but found NaN

This is a regression compared to v0.67.19
This doesn't happen with `float.NaN`</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

Fixes #3359

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.