Unit Testing missing assembly reference false-positive?

Trying to create unit tests for a collection of extension methods I'm making, but I get

[class] does not contain definition for ...

errors in VS if the assembly assembly is not included through it, and I get

The type or namespace name 'XenoRo' could not be found (are you missing a using directive or an assembly reference?)

in the editor when I do include the assembly.

Example:

Code (CSharp):

1. using NUnit.Framework;
2. using UnityEngine;
3.  
4. namespace Tests {
5.     public class Vector3ExtensionsTests {
6.         #region Methods
7.         [Test]
8.         public void WithXReturnsCorrectValue() {
9.             var vector = Random.onUnitSphere;
10.             var x = Random.value;
11.  
12.             var expected = vector;
13.             expected.x = x;
14.             var result = vector.WithX(x);
15.             Assert.That(result == expected);
16.         }
17.         #endregion
18.     }
19. }

...Errors on

.WithX(x)

and suggests fix by including assembly reference to

Assembly-CSharp-firstpass

, but if I tell VS to implemente the fix, it then errors in-editor saying that the namespace could not be found.

What is wrong? And how to fix?