Compilation error for nested private interface

[mssucksalot]

This can be compiled in old .NET

public class MyClass : MyClass.IMySubclass
{
    private interface IMySubclass
    {
        void ShowMe();
    }

    void IMySubclass.ShowMe()
    {
        Console.WriteLine("I am!");
    }

    public void Test()
    {
        IMySubclass mys = this;

        mys.ShowMe();
    }
}

It is supported at least since .NET Framework 4.8. I was not able to test earlier versions of VS like 2017, 2019.

However, H5 is oriented to .NET Core / .NET and at least C# version 7.2. So it must be supported.