csharplang-2372_1.cs

using System;
public class C {
   
    //Supported
    public static void func(char c){ }
    
    //Supported
    public static void func(ref char c){ }
    
    //Seems like this should also be allowed
    //public static void func(ref readonly char c){ }
    
    //Seems like this should also be allowed.
    //public static void func(in char c){ }
    
    //Seems like this should also be allowed..
    //public static void func(readonly char c){ }
    
    //Supported
    public static void funcS<T>(in T t) where T : struct{ }
    
    //Supported
    public static void funcSR<T>(ref T t) where T : class{ }
    
    //Supported
    public static void funcC<T>(in T t) where T : class{ }    
    
    //Supported
    public static void funcCR<T>(ref T t) where T : class{ }
}

​csharplang-2372_1.decompiled.cs

using System.Diagnostics;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
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 static void func(char c)
    {
    }

    public static void func(ref char c)
    {
    }

    public static void funcS<T>([In] [IsReadOnly] ref T t) where T : struct
    {
    }

    public static void funcSR<T>(ref T t) where T : class
    {
    }

    public static void funcC<T>([In] [IsReadOnly] ref T t) where T : class
    {
    }

    public static void funcCR<T>(ref T t) where T : class
    {
    }
}

​​.sharplab.json

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