0-readme.md

First note that Tagged is the type from here:

https://github.com/pointfreeco/swift-prelude/blob/master/Sources/Prelude/Tagged.swift

which has ExpressibleByIntegerLiteral when the wrapped value does.

1-optional-promotion-problem.swift

// This shows a problem with optional promotion:

import Prelude

enum Failure {}
typealias Failable<A> = Tagged<Failure, A?>
extension Tagged where Tag == Failure {
  var value: A {
    return self.unwrap
  }
}

enum Canceled {}
typealias Cancelable<A> = Tagged<Canceled, A?>
extension Tagged where Tag == Canceled {
  var notCanceled: A {
    return self.unwrap
  }
}

// This works
let result = Cancelable<Failable<Int>>(unwrap: Failable<Int>(unwrap: 2))

// This does not. Optional promotion doesn't seem to go deep enough.
//let result: Cancelable<Failable<Int>> = 2

2-expressible-fixes-it.swift

// This shows how to fix it:

// I make optionals be expressible:
extension Optional: ExpressibleByIntegerLiteral where Wrapped: ExpressibleByIntegerLiteral {
  public typealias IntegerLiteralType = Wrapped.IntegerLiteralType
  public init(integerLiteral value: Wrapped.IntegerLiteralType) {
    self = .some(.init(integerLiteral: value))
  }
} 

import Prelude

enum Failure {}
typealias Failable<A> = Tagged<Failure, A?>
extension Tagged where Tag == Failure {
  var value: A {
    return self.unwrap
  }
}

enum Canceled {}
typealias Cancelable<A> = Tagged<Canceled, A?>
extension Tagged where Tag == Canceled {
  var notCanceled: A {
    return self.unwrap
  }
}

// This works
let result = Cancelable<Failable<Int>>(unwrap: Failable<Int>(unwrap: 2))

// And now this works!
let result: Cancelable<Failable<Int>> = 2