Arities: 2, 3, 4, 5, 6

// OneOf<T1, T2> — binary outcome
OneOf<Error, User> result = TryGetUser(id);

// OneOf<T1, T2, T3> — three-way outcome
OneOf<ValidationError, NotFoundError, User> result = ValidateAndGet(id);

// OneOf<T1, T2, T3, T4> — four-way outcome (v1.12.0+)
OneOf<ValidationError, UnauthorizedError, NotFoundError, Order> result = GetOrder(id);

// OneOf<T1, T2, T3, T4, T5> — five-way outcome (v1.27.0+)
OneOf<ValidationError, UnauthorizedError, NotFoundError, ConflictError, Order> result = ProcessOrder(id);

// OneOf<T1, T2, T3, T4, T5, T6> — six-way outcome (v1.27.0+)
OneOf<ValidationError, UnauthorizedError, NotFoundError, ConflictError, ForbiddenError, Order> result = ComplexOp(id);

Chain extension methods convert between adjacent arities:

// Up-convert: add a new type slot (anchors the new type via defaultValue — not used at runtime)
OneOf<ValidationError, NotFoundError, User> three = ...;
OneOf<ValidationError, NotFoundError, User, ConflictError> four = three.ToFourWay(default(ConflictError));
OneOf<ValidationError, NotFoundError, User, ConflictError, ForbiddenError> five = four.ToFiveWay(default(ForbiddenError));

// Down-convert: narrows to lower arity — returns null if the dropped type slot was active
OneOf<ValidationError, NotFoundError, User>? narrowed = four.ToThreeWay();