Allow `satisfies` in string enum member initializers

🔍 Search Terms

satisfies, enum

✅ 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

I suggest that TypeScript should allow this:

enum Foo {
  Foo = "Foo" satisfies "Foo"
}

Currently this is treated as a computed enum member and requires number type, despite nothing being actually computed here.

📃 Motivating Example

With isolatedModules/isolatedDeclarations enabled references in enum members are restricted. For example, this does not compile with isolatedDeclarations:

const FOO = "Foo"

enum Foo { Foo = FOO }
// Error: Enum member initializers must be computable without references to external symbols with --isolatedDeclarations.

satisfies here will ensure that Foo is properly typed while not referring to external symbols:

const FOO = "Foo"

enum Foo { Foo = "Foo" satisfies typeof FOO }

💻 Use Cases

See motivating example above.

まず、satisfies を付けた場合と付けない場合の文字列 enum の例を再現し、issue で説明されている isolatedDeclarations のケースも含めます。次に、enum メンバーの初期化子がどのように分類・テストされるかを追跡し、受け入れられた文字列初期化子が意図した型の制約を維持しつつ、生成される JavaScript を変更しないことを検証します。