Microsoft/TypeScript

switch (typeof something) isn't seen as exhaustive

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#56,786 opened on Dec 14, 2023

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 (5 comments) (0 reactions) (0 assignees)TypeScript (6,726 forks)batch import
Domain: check: Control FlowHelp WantedPossible Improvement

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Description

🔎 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.

Contributor guide

Research direction
Inspect the control flow analysis for `typeof` narrowing in the TypeScript checker, specifically for generic types. Check the logic in `narrowTypeByTypeof` or related functions to ensure all typeof results are considered exhaustive when the switch covers all cases.
Tech stack
typescript
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScriptGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55