CS0535 'GeneratedMessagePackResolver.MyNamespace.ListOfIntFormatter' does not implement interface member 
'IMessagePackFormatter<ListOfInt>.Serialize(ref MessagePackWriter, ListOfInt, MessagePackSerializerOptions)'

CS0535 'GeneratedMessagePackResolver.MyNamespace.ListOfIntFormatter' does not implement interface member 
'IMessagePackFormatter<ListOfInt>.Deserialize(ref MessagePackReader, MessagePackSerializerOptions)'

[MessagePackObject(keyAsPropertyName: true)]
[MessagePackFormatter(typeof(ListOfIntMessagePackFormatter))]  // added after creation of ListOfIntMessagePackFormatter
public class ListOfInt : List<int>
{
    public ListOfInt() : base() { }
    public ListOfInt(IEnumerable<int> values) : base(values) { }
    public ListOfInt(int value) : base(value) { }
}

public class ListOfIntMessagePackFormatter: IMessagePackFormatter<ListOfInt>
{
// implemented .Serialize() and Deserialize()
}