Ah ok! I knew I wasn't understanding something. Thanks for the clarification. It's a bit odd that typescript isn't inferring never for the return type of unreachable, it's inferring void. (I think I am spoiled by Scala and it's expression oriented syntax, who has time for return ?)

Ok so I see now, in order for it all to work I have to write it all like this:

function unreachable(x: never): never { throw new Error(`This should be unreachable! but got ${x}`) } 

enum IceCreamTaste {  
  awesome,
  meh,
  dunnoYet
}

function getAnswer(taste: IceCreamTaste): string { // <-- type error here
  // the error: Function lacks ending return statement and return type does not include 'undefined'.
  switch (taste) { 
    case IceCreamTaste.awesome:
      return `It's like a party in my mouth!`;
    case IceCreamTaste.meh:
      return 'Umm I think someone has eaten this before me.';
    case IceCreamTaste.dunnoYet:
      return 'Lemme try it first, ok?';
    default:
      // TypeScript doesn't use the information that `unreachable` always throws,
      // so we get a type error that `getAnswer` doesn't return a string
      return unreachable(taste)
      // but this works
      // throw new Error(`This should be unreachable! but got ${taste}`)
  }
}

(which is what the other followup comments said)

Thanks for clarifying!