Anonymous Symbol Properties

### 🔍 Search Terms

"anonymous symbol properties"
"anonymous unique symbol"
"inline unique symbol"
"unique symbol property key"
"computed property unique symbol"
"symbol branding"

### ✅ Viability Checklist

- [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
- [x] This wouldn't change the runtime behavior of existing JavaScript code
- [x] This could be implemented without emitting different JS based on the types of the expressions
- [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
- [x] This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
- [x] This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

### ⭐ Suggestion

Allow `unique symbol` keys to be declared inline at the property declaration site.

### 📃 Motivating Example

**Current code:**
```ts
declare const brand: unique symbol;
type UserId = string & { readonly [brand]: "UserId" };
```

**Proposed code:**
```ts
type UserId = string & { readonly [brand: unique symbol]: "UserId" };
```

Each inline declaration would create a fresh symbol identity, equivalent to declaring a separate `unique symbol` and using it as the computed property key.

This is useful when a `unique symbol` property key is needed only once and does not need to be referenced independently. Common examples include phantom properties used for branded or refined types.

### 💻 Use Cases

1. What do you want to use this for?
Declaring single-use `unique symbol` property keys inline, particularly for phantom properties used by branded or refined types.

2. What shortcomings exist with current approaches?
The current approach requires introducing a value-level declaration solely to provide a unique identity for a type-level property key. For single-use symbol keys, this creates an otherwise unnecessary binding in the surrounding scope.

3. What workarounds are you using in the meantime?
Declaring a separate `unique symbol` value and using it as a computed property key.

Issue にある現在の型宣言と提案された型宣言から始め、TypeScript が計算されたプロパティキーと unique symbol のアイデンティティをどのように扱うかを追跡します。inline の unique symbol プロパティキーを受け入れ、各宣言に新しいアイデンティティを付与し、実行時の JavaScript 出力を維持できれば完了です。この issue では実装ファイルやテストは指定されていません。

索引モデルが issue の本文から書いたものです。