notnull constraint and value type

using System;
    // notnull constraint cannot handle both in and Nullable<T>.
    public interface IWriter<T> where T : notnull
    {
        void Write( in T o );
        void WriteNullable( in T? o );
    }

    public class ValueTypeWriter<T> : IWriter<T> where T : struct
    {
        public void Write( in T o )
        {
        }
        public void WriteNullable( in T? o )
        {
        }
    }

    // It's worse than that: notnull constraint cannot be "specialized" to handle nullable value types.
    public interface INullableWriter<T> where T : notnull
    {
        void WriteNullable( T? o );
    }

    public class NullableValueTypeWriter<T> : INullableWriter<T> where T : struct
    {
        public void WriteNullable( T? o )
        {
        }
    }



public class C {
    public void M() 
    {
        IWriter<int> w = new ValueTypeWriter<int>();
        w.Write( 8 );
        int? n = default;
        w.WriteNullable( in n );
    }
}