unique symbol lost on assignment to const despite type assertion

🔎 Search Terms

unique symbol widening

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground Link

💻 Code

const u = Symbol();
const a7 = u as typeof u // widened  to symbol

🙁 Actual behavior

a7 is symbol

🙂 Expected behavior

a7 has typeof u

Additional information about the issue

This issue is a huge problem for isolated declarations as it breaks the assumption that in an expression const o = a as T we can always use T as the type of o. This is also inconsistent with the way similar expressions work for string/number literal types (ex)

Similar issues:

• https://github.com/microsoft/TypeScript/issues/43657 - Widening on return type not on variable declaration
• https://github.com/microsoft/TypeScript/issues/55361 - Similar, explicitly typed variable is widened
• https://github.com/microsoft/TypeScript/issues/36876 - Widening when assigning from object property (but no assertion)
• https://github.com/microsoft/TypeScript/issues/53276

@Andarist perhaps https://github.com/microsoft/TypeScript/pull/54778 also fixes this?

リンクされている TypeScript Playground から始め、unique-symbol アサーションで推論される型を、対応する文字列リテラルおよび数値リテラルの場合と比較します。const 宣言と型アサーションに対するコンパイラーの widening の挙動を追跡し、関連するケースをリグレッションさせずに、アサーションされた unique-symbol 型が保持されることを確認します。