null_forgiving_CS8598_net_6_rc_1.cs

using System;
public class C {
    public void M(string value)
    {                
        if (value is null!) throw new ArgumentNullException(nameof(value));       
        
        // works in NET 5 (C#9) but fails in 6 RC1 (C#10) with error CS8598: The suppression operator is not allowed in this context        
        
        // I can see why as the null does not need a null foriving as string is not a declared string?
        // so it is correct to flag as a code smell, but should the error CS8598 be an error or a warning when migrating NET5 code?
    }
}

​null_forgiving_CS8598_net_6_rc_1.decompiled.cs

using System.Diagnostics;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Security;
using System.Security.Permissions;

[assembly: CompilationRelaxations(8)]
[assembly: RuntimeCompatibility(WrapNonExceptionThrows = true)]
[assembly: Debuggable(DebuggableAttribute.DebuggingModes.Default | DebuggableAttribute.DebuggingModes.DisableOptimizations | DebuggableAttribute.DebuggingModes.IgnoreSymbolStoreSequencePoints | DebuggableAttribute.DebuggingModes.EnableEditAndContinue)]
[assembly: SecurityPermission(SecurityAction.RequestMinimum, SkipVerification = true)]
[assembly: AssemblyVersion("0.0.0.0")]
[module: UnverifiableCode]
public class C
{
    public void M(string value)
    {
    }
}

​​.sharplab.json

{
    "version": 1,
    "target": "C#",
    "mode": "Debug",
    "branch": "main"
}