Microsoft/TypeScript

Exhaustive switch case with returns can mark a finally block as unreachable

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#61,259 opened on Feb 24, 2025

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BugDomain: check: Control FlowHelp Wanted

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Description

🔎 Search Terms

finally switch exhaustive unreachable

🕗 Version & Regression Information

This is the behavior in every version I tried ( doesn't typecheck pre 3.7 ), and I reviewed the FAQ for entries about unreachable code

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYewdgzgLgBAhjAvDAFANwFwwIwwD4wBMAlEgHwwDeAUDDFAE4CeVtdMEA7gJZTAAWqNKRrt2wOBACmOLAylQArgzAw0bMROlE5C5avViAvmyMwAZtzBwANjZajx4CCBtSAdDZABzFAHIpAA9eP2JTahNqUEhYACMkISwwRQBbWKkGUkQKR0YHDQ4ePkF0EQK6LRlsXSUVNXKYABMpczhFGyga-XrjUwsrW3tWTWdXDy9fAOCoUPDI6OgYYAT0LFwCEnJh+mZtui5eASEysQrJKq66w1PKnRh5WoMCkzozS2s7fLFuc1RGRSkJxuozcnh8-iCITCvVeEWoUWcsEaK0wMGSaQyWRybDye0KhxKwjxZ201XueiuDWarXanXJjx67BeMDeA0+eJ+fwYAKBI0gYzBk0hM2hTLmQA

💻 Code

const c = (v: 1 | 2) => {
  try {
    switch (v) {
      case 1: return v
      case 2: return v
    }
  } finally {
    if (true) {
      console.log('exit')
    }
  }
}

🙁 Actual behavior

The contents of the finally blocked are marked as unreachable

🙂 Expected behavior

That they are reachable

Additional information about the issue

While the example seems to include some confusing additions the replication apparently requires:

  • The cases to be exhaustive
  • No default case
  • For the switch expression be typed as a literal union of some form
  • An if statement to be in the finally block

I assume this is a chaotic mix of unreachable patterns upsetting flow analysis. I've also noticed that it you add an unreachable return statement after the switch then the contents of finally become reachable.

Contributor guide

Research direction
Investigate the control flow analysis for exhaustive switch statements with return statements inside try finally. Check how the compiler determines unreachable code in the finally block. Look at the flow graph generation for try statements and switch statements.
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.
1-3 hours
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.
50