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.

Start by running the provided TypeScript Playground reproduction and compare the generic function with the any version. Trace how typeof narrowing and switch exhaustiveness are handled for generic types, then add a regression test showing that all typeof cases satisfy the return type without TS2366.