Swift Anonymous Enum and automatic case-flattening (Thought experiment) https://twitter.com/inamiy/status/1552162547079725057

swift-anonymous-enum-case-flattening.swift

//---------------------------------------------------
// Anonymous 2-case enum example
//---------------------------------------------------

struct Error0: Swift.Error {}
struct Error1: Swift.Error {}

// Here, new `|` symbol is used as an anonymous enum type.
typealias Error01 = Error0 | Error1

// is equivalent to:
// NOTE: automatically tagged by number (like tuple)
enum Error01 {
    case 0(Error0)
    case 1(Error1)
}

// Pattern match example.
let error01: Error01 = ...
switch error01 {
    case let .0(error0): ...
    case let .1(error1): ...
}

//---------------------------------------------------
// 3-case anonymous enum
//---------------------------------------------------

typealias Error012 = Error0 | Error1 | Error2

// is equivalent to:
enum Error012 {
    case 0(Error0)
    case 1(Error1)
    case 2(Error2)
}

let error012: Error012 = ...
switch error01 {
    case let .0(error0): ...
    case let .1(error1): ...
    case let .2(error2): ...
}

//---------------------------------------------------
// Q. What about `Error01 | Error2` ?
//---------------------------------------------------

typealias Error01_2 = Error01 | Error2

// is NOT equivalent to:
enum Error01_2 {
    case 0(Error01)
    case 1(Error2)
}

// but instead `case`s are automatically flattened as follows: 
enum Error01_2 {
    case 0(Error0)
    case 1(Error1)
    case 2(Error2)
}

// This special rule will only apply when one of type with "multiple `case`s" are combined.
//
// This rule is "required" because in general,
// `Error01_2 == Error01 | Error2 == Error0 | Error1 | Error2` should hold.