Unreachable code breaks subsequent CFA

[Sainan]

🔎 Search Terms

control flow analysis, unreachable code

🕗 Version & Regression Information

Tested in 5.8.2

⏯ Playground Link

https://www.typescriptlang.org/play/?allowUnreachableCode=true#code/GYVwdgxgLglg9mABFApgZygCgIYC5FggC2ARigE6IA+i4AJisDGCnQJT4BucMdiA3gChEiGMESYo5ECjYDhIxORRQQ5MAG4FAXwViJ2RAEIAvCdpgGTFu3mLEEBBkQl8hUhUTnsW+47BocAA2KAB0QXAA5pgkbL6IutpAA

💻 Code

function test(a: number | undefined): void {
  if (true) {
    return;
  }
  if (a !== undefined) {
    const b: number = a;
    console.log(b);
  }
}

🙁 Actual behavior

Type 'number | undefined' is not assignable to type 'number'.
  Type 'undefined' is not assignable to type 'number'.

🙂 Expected behavior

Code compiles without error.

Additional information about the issue

No response

[MartinJohns]

Duplicate of #26914.