[GenerateOneOf] why must the generated class be top-level (see error ONEOFGEN001)

[anemitoff]

My title says it all....

I would like to write code such as

public abstact class FooQuery {
  public record Query;
  [GenerateOneOf] public partial class QueryResult : OneOfBase<Foo[], SomeError>;
}

I initially thought ONEOFGEN001 was produced by C# compiler but after taking another look I see it comes from line 124 of OneOfGenerator.cs