Custom formatter on nullable value type doesn't work

[aradalvand]

Given this Program.cs:

using MemoryPack;
using NodaTime;

Console.WriteLine("Hello, World");

[MemoryPackable]
public partial record TestRequest(
    [property: InstantFormatter]
    Instant Instant1,
    [property: InstantFormatter]
    Instant? Instant2
);

[AttributeUsage(AttributeTargets.All)]
public class InstantFormatter : MemoryPackCustomFormatterAttribute<Instant>
{
    public override IMemoryPackFormatter<Instant> GetFormatter() => new Formatter();

    private class Formatter : MemoryPackFormatter<Instant>
    {
        public override void Serialize<TBufferWriter>(ref MemoryPackWriter<TBufferWriter> writer, scoped ref Instant value)
        {
            writer.WriteValue(value.ToUnixTimeTicks());
        }

        public override void Deserialize(ref MemoryPackReader reader, scoped ref Instant value)
        {
            value = Instant.FromUnixTimeTicks(reader.ReadValue<long>());
        }
    }
}

The build fails:

TestRequest.MemoryPackFormatter.g.cs(34,91): error CS0266: Cannot implicitly convert type 'MemoryPack.IMemoryPackFormatter<NodaTime.Instant>' to 'MemoryPack.IMemoryPackFormatter<NodaTime.Instant?>'. An explicit conversion exists (are you missing a cast?)

Ideally, I shouldn't have to repeat the code in MemoryPack's NullableFormatter<T> + have separate nullable and non-nullable formatters for every value type — this is bad composability. MemoryPack should be smart enough in this example to know that formatter I specified should be wrapped in a NullableFormatter<T>.

Worth noting that MessagePack works this way.