exhaustive-switch.ts

function exhaustive(x: never): never {
  throw new Error(`
    Unreachable default case.
    ${x} is not assignable to type never.
  `)
}

type Fruit = "apple" | "banana" | "mango";

declare const fruit: Fruit;

switch (fruit) {
  case 'apple':
    break;
  case 'banana':
    break;
  default: exhaustive(fruit); // Argument of type '"mango"' is not assignable to parameter of type 'never'.
}

https://www.typescriptlang.org/play/#code/GYVwdgxgLglg9mABAUwB4AsCGIDOsBuyAFKgFyJjKEBOAlOZTYgN4BQiiU61cA7hcn4BRaj2pEABuw6IAqmGrJMELACMANskQATZMGzqoiCJhzIAdNI4ASZqgC+iGDgpwjpnDADmYTBq1QcJwAngAOWozI1JYcErSs9qysUGFaAGLUIDBGALyIAESYoaGa+YgAPgWqmL6+ZZX5ALY1XnD5ANxJuhDqmIrGCHiIwJnZ5BlZUJ2sOLzZKohEI5O0LNImZogA5EUlyFukVoiqipgA1p0cG1pb1bWYB0cnShfSuvoghuRoWLgExMtsrROvYgA