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

🔎 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.

リンクされた TypeScript Playground の再現コードから始め、try/finally 内の網羅的な literal-union switch を、列挙されたバリエーション、つまり default case または switch の後にある到達不能な return と比較します。if 文を finally 内で到達不能と判定するフロー解析を追跡し、その後、finally の内容が到達可能であることを示すカバレッジを追加して、期待される diagnostics を検証します。