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?

Bắt đầu với TypeScript Playground được liên kết và so sánh các kiểu được suy luận cho phép assertion unique-symbol với các trường hợp tương tự của literal chuỗi và số. Theo dõi hành vi widening của compiler đối với các khai báo const và type assertion, sau đó xác minh rằng kiểu unique-symbol được assertion vẫn được giữ nguyên mà không gây hồi quy cho các trường hợp liên quan.