Define what the generators do with a union or a closed class, through IsUnion and IsClosed #223

What you're trying to do

Use a C# 15 union or a closed class next to TypeKit's value objects, in a project woven by Metalama 2027 (C# 15 on net11.0), and get either a working value object or a clear diagnostic. Never one that compiles and does less than it appears to.

Two shapes have no defined answer today, because Metalama 2026.1 can't compile them in a woven project (decision 120):

1. A value object that wraps a union or a closed type: readonly partial record struct PetId : IValue<Pet>; where Pet is a union. The generated JSON goes through GeneratedJsonPlan<T>, and System.Text.Json has union support (net11.0: Option/Result refusal and value-object JSON under System.Text.Json's union support #213). A union has no IParsable, no IComparable and no type converter of its own.
2. A union or a closed class that claims a TypeKit contract, for example a union declared with IValue<int> or IClosedSet<TSelf>, if C# 15 lets a union declare interfaces at all.

What you'd like

• Metalama 2027.0's INamedType.IsUnion, IsClosed and IUnionFacet (release notes, "Unions" and "Closed classes") let the fabric and the aspects tell these shapes apart. That's the code model's answer, not a name.
• For each shape, decide: generated and tested (the JSON byte for byte as the wrapped union's), or refused with a CMTK diagnostic that names the shape. A new diagnostic ships at Warning, since a minor never adds an Error (decision 34).
• Rows for unions and closed types join the generated-type axes, on the net11.0 test run.

Needs #173 and the net11.0 target (#208).

Does it change something that compiles or works today?

No. Neither shape compiles in a woven project before 2.4.0, which moves to Metalama 2027.0 (decision 129). That is why this is 2.4.0 work: the release that makes them compile defines what they do.