[mpc] Fix collecting closed type generics

[waitxd]

Most of work was done in #830.
However, current implementation fails on this code:

[MessagePackObject]
public class CustomGenericClass<T>
{
    [Key(1)] public List<T> ListOfT;
}

[MessagePackObject]
public class ClassToSerialize
{
    [Key(0)] public CustomGenericClass<string> GenericField;
}

Generated code

        static GeneratedResolverGetFormatterHelper()
        {
            lookup = new global::System.Collections.Generic.Dictionary<Type, int>(3)
            {
                { typeof(global::CustomGenericClass<string>), 0 },
                { typeof(global::System.Collections.Generic.List<T>), 1 },
                { typeof(global::ClassToSerialize), 2 },
            };
        }

and

            switch (key)
            {
                case 0: return new MessagePack.Formatters.CustomGenericClassFormatter<string>();
                case 1: return new global::MessagePack.Formatters.ListFormatter<T>();
                case 2: return new MessagePack.Formatters.ClassToSerializeFormatter();
                default: return null;
            }

MPC is trying to collect type argument T from open generic type CustomGenericClass<T> here:
https://github.com/neuecc/MessagePack-CSharp/blob/275eb96cdcce5d10c3326a2b940af5fcdeeda066/src/MessagePack.GeneratorCore/CodeAnalysis/TypeCollector.cs#L544-L548
But because type argument has no declared accessibility (Accessibility.NotApplicable) it fails to pass IsAllowAccessibility check and collect stops there.

We can skip all generics definitions (e.g. Foo<T>) and collect only closed type references (e.g. Foo<string>). That's a main purpose of this commit.

Generated code

        static GeneratedResolverGetFormatterHelper()
        {
            lookup = new global::System.Collections.Generic.Dictionary<Type, int>(3)
            {
                { typeof(global::CustomGenericClass<string>), 0 },
                { typeof(global::System.Collections.Generic.List<string>), 1 },
                { typeof(global::ClassToSerialize), 2 },
            };
        }

and

            switch (key)
            {
                case 0: return new MessagePack.Formatters.CustomGenericClassFormatter<string>();
                case 1: return new global::MessagePack.Formatters.ListFormatter<string>();
                case 2: return new MessagePack.Formatters.ClassToSerializeFormatter();
                default: return null;
            }

[neuecc]

Thanks, seems very good!

[AArnott]

I see no problems either. I'll leave it to @neuecc to complete the PR since mpc is really his area.

[AArnott]

Can we close #448 when this is merged?

[waitxd]

Can we close #448 when this is merged?

I think to finalize generics support in mpc we should decide do we need to support such cases:

[MessagePackObject]
public class Foo<T>
{
}

var resolver = MessagePack.Resolvers.CompositeResolver.Create(
    MessagePack.Resolvers.GeneratedResolver.Instance,
    MessagePack.Resolvers.BuiltinResolver.Instance
);
var options = MessagePackSerializerOptions.Standard.WithResolver(resolver);

var foo = new Foo<string>();
var serializedValue = MessagePackSerializer.Serialize(foo, options);

Right now mpc is unable to resolve type attributes (T) for types which are not referenced somewhere in types with MessagePackObject attribute.

So we should update docs and reflect this as known issue or extend syntax analysis (either via custom attribute or code scanning) to support such cases.

[neuecc]

I'm against spreading the analysis across the board, so it would be a document.
Anyway, I'll merge this one, thanks!