MsgPack013 Inaccessible formatter instance

A custom IMessagePackFormatter<T> implementation must have an accessible instance to be included in the source generated resolver.

This instance can be exposed by any of the following:

A public default constructor.
A public static readonly field called Instance.

Examples of patterns that are flagged by this analyzer

class CustomBFormatter : IMessagePackFormatter<B> { // MsgPack013
    private CustomBFormatter { } // hides the default public constructor

    void Serialize(ref MessagePackWriter writer, B value, MessagePackSerializerOptions options)
        => throw new NotImplementedException();

    B Deserialize(ref MessagePackReader reader, MessagePackSerializerOptions options)
        => throw new NotImplementedException();
}

Typical fix

Delete the non-public default constructor so that C# can access it:

class CustomBFormatter : IMessagePackFormatter<B> {
    void Serialize(ref MessagePackWriter writer, B value, MessagePackSerializerOptions options)
        => throw new NotImplementedException();

    B Deserialize(ref MessagePackReader reader, MessagePackSerializerOptions options)
        => throw new NotImplementedException();
}

Or make a singleton instance available:

class CustomBFormatter : IMessagePackFormatter<B> {
    public static readonly CustomBFormatter Instance = new();

    private CustomBFormatter { } // hides the default public constructor

    void Serialize(ref MessagePackWriter writer, B value, MessagePackSerializerOptions options)
        => throw new NotImplementedException();

    B Deserialize(ref MessagePackReader reader, MessagePackSerializerOptions options)
        => throw new NotImplementedException();
}