[API Proposal]: UnionAttribute and IUnion interface

[AlekseyTs]

Background and motivation

As part of supporting Unions feature, we need:

• a new attribute that will be used by C# compiler or by a C# developer to designate a Union type.
• a new interface that will be implemented by compiler generated or user defined Union types.

API Proposal

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(Class | Struct, AllowMultiple = false)]
    public class UnionAttribute : Attribute;

    public interface IUnion
    {
        // The value of the union or null
        object? Value { get; }
    }
}

API Usage

public union Pet(Cat, Dog){ ... }  // C# compiler generates a struct implementing IUnion with UnionAttribute on the struct

[Union] public struct Pet : IUnion // Developer manually declares a union type
{
    public Pet(Cat value) => Value = value;
    public Pet(Dog value) => Value = value;
    
    public object? Value { get; }
    
    ... 
}

Alternative Designs

No response

Risks

No response

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[tannergooding]

There may be some discussion on namespace since users are allowed to specify IUnion and/or [Union] on their own types.