Switch statements exhaustiveness check is failing when the expression has generic type but with literal type constraint

[jfet97]

🔎 Search Terms

switch exhaustiveness literals

🕗 Version & Regression Information

This is mostly related with the new capabilities from #56941, so I'd say TS 5.8.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20241202#code/C4TwDgpgBACghiANgezgEygXgFBSgHygG8oB9YOAcwC4oAiOAV2AAs6BuKRgZwgCcAdnAC2EWt2B8AlgMqcwcbtwDuyPmnGSZlKAF9cBYmQo16AYzh9gHKFOARh3WgEE+fBAB4SUjVAnTZTgBHRjgBYDsQWgFGYQAjfj0APj0DQhJyKlo6SD5uZAEbIVFNALkoKjEoGPjE3XZsbFBIWAQUdABhAvtwrGIDAG0YWwFWpFQMRVgBukzKOgBdBdoAeWE7DxgAGno5uiSG+saAM0YBMwiCqGFkMwBreHH0TdIAFWcAcSgIAA8etG4Y3aaBmewWSQAFAZ-LQYG9PltsABKWFtCZdcIQcJDeEfBb9PAqOxmFhQCH+JEEvBQCy8ehMVh0agGalQPgQYCMQRGHj8YpVOj2CR0HYKJSqdTZdDrQp6Bqs2nQOgWKxMlnU9mc7neeyOWgDby+OgARgATABmEVQEJhCKgWhm816fFHBWKJW5fKFZmsvCarmjEj8qUCNDsuBWyq0U0Adjl6oA9AmoGgIMcmIhgD7fUmoKw+MhlNUIEWAKJuNQQuhnO4CQujEx0JHy1K6IA

💻 Code

type Payload =
  | { _tag: "auth"; username: string; password: string }
  | { _tag: "cart"; items: Array<{ id: string; quantity: number }> }
  | { _tag: "person"; name: string; age: number };

type PayloadContent = {
  [P in Payload as P["_tag"]]: Omit<P, "_tag">;
};

function mockPayload<P_TAG extends Payload["_tag"]>(
  str: P_TAG,
): PayloadContent[P_TAG] {
  switch (str) {
    case "auth":
      return { username: "test", password: "admin" };
    case "cart":
      return { items: [{ id: "123", quantity: 123 }] };
    case "person":
      return { name: "andrea", age: 27 };
    // default:
      // throw new Error("unknown tag");
  }
}

🙁 Actual behavior

Error: Function lacks ending return statement and return type does not include 'undefined'.

🙂 Expected behavior

The exhaustiveness check should understand that there is no missing case to handle, so the default case should be optional.

Additional information about the issue

It's the companion issue for PR #60644 that will fix this if merged.