Function indicating that this part is unreachable.

For example, the following code passed type checking.

import { unreachable } from "./mod.ts";

type Animal = "dog" | "cat";

function say(animal: Animal): void {
  switch (animal) {
    case "dog":
      console.log("dog");
      break;
    case "cat":
      console.log("dog");
      break;
    default:
      unreachable(animal);
  }
}
say("dog");

But the following code because a case for "bird" is missing.

import { unreachable } from "./mod.ts";

type Animal = "dog" | "cat" | "bird";

function say(animal: Animal): void {
  switch (animal) {
    case "dog":
      console.log("dog");
      break;
    case "cat":
      console.log("dog");
      break;
    default:
      unreachable(animal);
  }
}
say("dog");