`isolatedModules` does not generate the same code

🔎 Search Terms

isolatedModules
const enum
inlining
generated code

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?preserveConstEnums=true&module=5&isolatedModules=true&ts=5.7.0-dev.20240930#code/FAYw9gdgzgLgBAUwgVwLZwHJhgZTAQQHcEoxUEBhSWAURXQG9g449yA1AQwBtkEAaZnHwRsACwQAnAPIQEwAL7BlAM2QQQMAJaQ4AEzClyMMVogBzABSxOMBAC5M2PERJlK1GHTQBKRwDcwLT04JhYoQi0YEDE4axhbBB9QoRYQTigEJ1wCYiMPaC96ADo2BC5eB1SWOHBoMG4EYu4wKwByMoq+Np8AbmqWACNJBE4Aa36a2oysrBzXfKpC71RikXEpWSqptOoGppb29ZNNuR7JneHRiaElBSA

💻 Code

const enum NotSoAwesomeConstEnum {
  SomeValue,
  AnotherOne
}


function dosomething(state: NotSoAwesomeConstEnum): void {
  switch (state) {
    case NotSoAwesomeConstEnum.SomeValue:
      console.log('SomeValue');
      break;
    case NotSoAwesomeConstEnum.AnotherOne:
      console.log('AnotherOne');
      break;
  }
}

🙁 Actual behavior

The emitted code is not the same when using isolatedModules.

🙂 Expected behavior

The emitted code should be the same when using isolatedModules as it is described as nothing else but a check

Setting the isolatedModules flag tells TypeScript to warn you if you write certain code that can’t be correctly interpreted by a single-file transpilation process.

const enum values should be inlined even when using this flag. (Or an error should be emitted).

Additional information about the issue

From the documentation:

It does not change the behavior of your code, or otherwise change the behavior of TypeScript’s checking and emitting process.

Which is not true, at least for const enum as this prevents inlining.
See the playground link and https://github.com/microsoft/TypeScript/issues/16671#issuecomment-1686858424

リンクされた TypeScript Playground から始め、const enum の例について isolatedModules を指定した場合と指定しない場合の出力コードを比較します。const enum の処理を担当するコンパイラーパスを追跡し、値をインライン化すべきか、エラーを報告すべきかを判断します。出力が一致するか、明確な診断が得られれば完了です。