Question - Testing codefix that also fixes issue

[leus]

Trying to implement a code fix to help writing manual mapping code.

This is my test case:

var test = @"
    using System;
    using System.Xml.Serialization;
    namespace ConsoleApplication1
    {
    #pragma warning disable CS1003
    #pragma warning disable CS1002
        interface IMessage { }
        class LeftClass : IMessage { }
        [System.Xml.Serialization.XmlTypeAttribute(Namespace=""http://www.mismo.org/residential/2009/schemas"")]
        class RightClass { }
        class Something { public LeftClass Prop { get; set; } }

        static class Test {
            public static void TestMethod()
            {
                var a = new RightClass();
                var b = new Something {
                    Prop = a
                };
            }
        }
    #pragma warning restore CS1003
    #pragma warning restore CS1002
    }";

The line Prop = a fails because the right side is of a different type. However, since it has a known annotation, I want to propose the creation of an extension method to transform to the left type:

            var fixtest = @"using System;
using System.Xml.Serialization;

namespace ConsoleApplication1
{
#pragma warning disable CS1003
#pragma warning disable CS1002
    interface IMessage
    {
    }

    class LeftClass : IMessage
    {
    }

    [System.CodeDom.Compiler.GeneratedCodeAttribute(""xsd"", ""4.8.3928.0"")]
    [System.SerializableAttribute()]
    [System.Diagnostics.DebuggerStepThroughAttribute()]
    [System.ComponentModel.DesignerCategoryAttribute(""code"")]
    [System.Xml.Serialization.XmlTypeAttribute(Namespace = ""http://www.mismo.org/residential/2009/schemas"")]
    [System.Xml.Serialization.XmlRootAttribute(Namespace = ""http://www.mismo.org/residential/2009/schemas"", IsNullable = false)]
    class RightClass
    {
    }

    class Something
    {
        public LeftClass Prop { get; set; }
    }

    static class Test
    {
        public static void TestMethod()
        {
            var a = new RightClass();
            var b = new Something
            {
                Prop = a.ToDomain()
            };
        }

        private static LeftClass ToDomain(this RightClass source) => throw new NotImplementedException();
    }
#pragma warning restore CS1003
#pragma warning restore CS1002
}";

Now, I have the code and it works. However, testing is proving difficult, since the code fix actually eliminates both diagnostics.

            var expected = new[]
            {
                // /0/Test0.cs(24,33): warning WILQO001: Class initialization assignment to IMessage detected
                VerifyCS.Diagnostic().WithSpan(24, 33, 24, 41),
                // /0/Test0.cs(24,40): error CS0029: Cannot implicitly convert type 'ConsoleApplication1.RightClass' to 'ConsoleApplication1.LeftClass'
                DiagnosticResult.CompilerError("CS0029").WithSpan(24, 40, 24, 41).WithArguments("ConsoleApplication1.RightClass", "ConsoleApplication1.LeftClass"),
            };

            var analyzerTest = new CSharpAnalyzerTest<MessageAssignmentAnalyzer, DefaultVerifier>
            {
                TestState = { Sources = { test } }
            };
            analyzerTest.TestState.ExpectedDiagnostics.AddRange(expected);
            await analyzerTest.RunAsync(); // works

            var codefixTest = new CSharpCodeFixTest<MessageAssignmentAnalyzer, MessageAssignmentCodeFixProvider, DefaultVerifier>
            {
                TestState = { Sources = { test } },
                FixedState = { Sources = { fixtest } },
            };
            codefixTest.TestState.ExpectedDiagnostics.AddRange(expected);
            await codefixTest.RunAsync();

If I add diagnostics to the FixedState member it fails because it actually is returning no diagnostics; but if I leave the FixedState.Diagnostics collection empty, the test code "helpfully" calculates 1 diagnostic expected.

Any setting that I need to change to allow for the diagnostics after the fix to be empty?