In particular, in use cases with AOT, the CompositeResolver is essential.
However, there are performance penalties,
such as the need for dictionary lookups every time GetFormatter is called,
as well as issues with unstable behavior in some AOT environments (e.g., Unity's IL2CPP).

How about supporting the generation of CompositeResolver with Source Generators?

[GeneratedMessagePackResolver(typeof(FooGeneated.Instance, StandardResolver.Instance))]
public partial class MyAppResolver;

It would be preferable to reconcile naming conventions and such after #1691.