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?

Commencez par le TypeScript Playground lié et comparez les types inférés pour l’assertion unique-symbol avec les cas analogues de littéraux de chaîne et de nombre. Suivez le comportement d’élargissement du compilateur pour les déclarations const et les assertions de type, puis vérifiez que le type unique-symbol déclaré est préservé sans provoquer de régression dans les cas associés.