Inconsistent and uncompilable decompilation for a combination of null forgiving operator and extension method usage

[coxantohi]

Input code

public static class ExtensionMethods
{
    public static int Count(this System.Collections.IEnumerable source)
    {
        return 5;
    }
}
public class ProblemClass
{
    public void ProblemMethod()
    {
        IEnumerable<string> s = new List<string> { "ABC", "DEF" };
        int len = s != null ? Enumerable.Count(s) : 0;
    }
}

Decompiled code

`ExtensionMethods.cs'

using System.Collections;

public static class ExtensionMethods
{
	public static int Count(this IEnumerable source)
	{
		return 5;
	}
}

`ProblemClass.cs'

using System.Collections.Generic;
using System.Linq;

public class ProblemClass
{
	public void ProblemMethod()
	{
		int len = Enumerable.Count(new List<string> { "ABC", "DEF" }?) ?? 0;
	}
}

The decompiled code is not compilable.

When unchecking the decompiler option C# 6.0 / VS 2015: Decompile ?. and ?[] operators or the decompiler option C# 3.0 / VS 2008: Use extension method syntax the saved code in ProblemClass.cs will be compilable:

using System.Collections.Generic;
using System.Linq;

public class ProblemClass
{
	public void ProblemMethod()
	{
		IEnumerable<string> s = new List<string> { "ABC", "DEF" };
		int len = ((s != null) ? Enumerable.Count(s) : 0);
	}
}

Another manifestation of the error is that when switching the focus from the method to the class the decompiled code will change (gif):

Details

• Product in use: ILSpy version 8.2.0.7535
• Version in use: .NET version 6.0.28-servicing.24120.7+34a109148c7d8a2c8e6431e83e4bce5712dd8083

ICSharpCode.Decompiler.CSharp.Resolver.CSharpResolver.GetAllExtensionMethods will return different results when focusing on the method in contrast with when focusing on the class.

When focusing on the method the ICSharpCode.Decompiler.CSharp.Transforms.IntroduceExtensionMethodsCanTransformToExtensionMethodCall method will return false because of the method.Equals(or.GetBestCandidateWithSubstitutedTypeArguments()) subcondition.

The issue was found when trying to decompile Microsoft.Graph.Communications.Common.
The extension method causing the confusion is Microsoft.Graph.Communications.Common.Validator.Any:

/// <summary>
/// Determines whether a sequence contains any elements.
/// </summary>
/// <param name="source">The <see cref="T:System.Collections.IEnumerable" /> to check for emptiness.</param>
/// <returns>
/// <see langword="true" /> if the source sequence contains any elements; otherwise, <see langword="false" />.
/// </returns>
public static bool Any(this IEnumerable source)
{
	if (source == null)
	{
		return false;
	}
	return source.Cast<object>()?.Any((object x) => x != null) == true;
}

The methods failing to be correctly decompiled are Microsoft.Graph.Communications.Common.Transport.HttpRequestMessageExtensions.ToGraphRequestAsync<T> and Microsoft.Graph.Communications.Common.Transport.HttpResponseMessageExtensions.ToGraphResponseAsync<T>.