[isolatedDeclarations][5.5] Type annotations needed for literals containing enum values

🔍 Search Terms

enum, isolated declarations

✅ Viability Checklist

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

⭐ Suggestion

Enum values seem trivially inferrable if the enum is defined in the lexical scope, so isolated declarations shouldn't report them as errors.

📃 Motivating Example

export enum Foo {
  A = 1,
  B = 2,
}

export const x = Foo.A as const;
             ~
export const y = [Foo.A] as const;
                  ~~~~~

Playground: https://tsplay.dev/w68Prm

💻 Use Cases

Self-apparent

Issue の TypeScript Playground の例に示されている isolatedDeclarations の動作から始め、コンパイラーの isolated-declaration 診断と型チェックのエントリーポイントを調べます。export されたリテラルとタプル要素について enum の値がどのように推論されるかを確認します。完了の条件は、示されている Foo.A の例でエラーが報告されなくなり、有効な declaration output が維持されることです。