switch (typeof something) isn't seen as exhaustive

🔎 Search Terms

• typeof
• switch
• narrow
• type guard

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Type Guards

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.3.2#code/GYVwdgxgLglg9mABBAFgUwgawIIBtcA8AKgHwAUAznALZpQoxgDmAXIkQJRtGIDeAUIkQUA7jCipEZKAE8ADmjjBhNOg2Yc+gocgCGFNIgBEYENQBGaAE5GW2oRH2Gj5mE0ZRb9vQePm4cLhoumBeOj7OFDIWgWE6jr5G4AAmaMCMaMlxDk7GoJCwCNkRxnDmAFYYnnbhCZFQVoxMxUJWdCBWSFS09E0A3NoAvvyDQA

💻 Code

function checkAll<T>(something: T): T {
  switch (typeof something) {
    case "number":
    case "bigint":
    case "boolean":
    case "symbol":
    case "undefined":
    case "function":
    case "object":
    case "string":
      return something;
  }
}

🙁 Actual behavior

This emits the TS error: "Function lacks ending return statement and return type does not include 'undefined'.(2366)"

🙂 Expected behavior

No error, since all possible results of typeof are handled.

Additional information about the issue

It seems to only be an issue when the function argument is generic. It works if I switch the return type and parameter types to any, but I'm not sure that's a fair comparison.

まず、提供されている TypeScript Playground の再現コードを実行し、ジェネリック関数と any 版を比較します。ジェネリック型に対して typeof による絞り込みと switch の網羅性がどのように扱われるかを追跡し、その後、すべての typeof ケースが TS2366 なしで戻り値の型を満たすことを示す回帰テストを追加します。