[NO-MERGE] Prototype: C# union types, closed enums, and closed hierarchies

[eiriktsarpalis]

⚠️ NO-MERGE — Prototype / Design Exploration

This PR is a design prototype for mapping upcoming C# language features to PolyType's type shape abstractions. It is not intended to be merged in its current form — it exists to validate API feasibility and gather feedback.

Background

C# is introducing three related language features:

• Union types — tagged unions with structural case types
• Closed enums — enums that cannot be extended
• Closed hierarchies — class/interface hierarchies with a fixed set of subtypes

See also the System.Text.Json design for the same features.

API Changes

New types

Type	Description
UnionKind enum	Distinguishes ClassHierarchy, FSharpUnion, and CSharpUnion union origins
DelegateMarshaler<TSource, TTarget>	Constraint-free IMarshaler using Func delegates, enabling marshal/unmarshal between unrelated types

Extended interfaces

Interface	New member	Description
IUnionTypeShape	UnionKind UnionKind	Identifies the kind of union (class hierarchy, F# DU, or C# union)
IEnumTypeShape	bool IsClosed	Whether the enum is marked [Closed]

Extended attributes

Attribute	New property	Description
TypeShapeAttribute	bool InferDerivedTypes	Opt-in for auto-discovering derived types from [ClosedSubtype] attributes or assembly scanning
GenerateShapeAttribute	bool InferDerivedTypes	Same, for source-generated shapes
GenerateShapeForAttribute	bool InferDerivedTypes	Same, for external type shapes

How C# Union Types Map to IUnionTypeShape

The key insight is that IUnionCaseShape<TUnionCase, TUnion> has no TUnionCase : TUnion constraint at the interface level. This means PolyType already supports the case where union case types are unrelated to the union type — exactly what C# unions require.

C# union Pet                    PolyType mapping
─────────────────               ──────────────────────────────
union Pet {                     IUnionTypeShape<Pet>
  Dog(string Name, int Age);      UnionKind = CSharpUnion
  Cat(string Name, bool Indoor);  UnionCases[0] = IUnionCaseShape<Dog, Pet>
  int;                            UnionCases[1] = IUnionCaseShape<Cat, Pet>
}                                 UnionCases[2] = IUnionCaseShape<int, Pet>

Marshaling: DelegateMarshaler<Dog, Pet> wraps:

• Marshal: calls new Pet(dog) (union constructor)
• Unmarshal: calls ((IUnion)pet).Value and casts to Dog

Case index: determined by checking the runtime type of IUnion.Value.

Detection: by metadata name (System.Runtime.CompilerServices.UnionAttribute + System.IUnion), not by PolyType-defined types. Mock attributes are provided in test cases since no compiler supports these yet.

Closed Hierarchy Support

[ClosedSubtype(typeof(Circle))] attributes on a base type are always honored for union detection (similar to [DerivedTypeShapeAttribute]). Assembly scanning (finding all subtypes in the same assembly) requires explicit InferDerivedTypes = true opt-in.

What's included

• Reflection provider: full C# union, closed enum, and closed hierarchy support
• Source generator: closed enum IsClosed and [ClosedSubtype]-based union detection
• Example JSON serializer: structural matching converter for C# unions (no discriminator — matches by JSON token type and property names)
• 21 new tests, 273,600 total tests pass across net10.0/net9.0/net8.0/net472