Allow accessor properties in .d.ts even when targeting ES5

[nicolo-ribaudo]

🔍 Search Terms

Properties with the 'accessor' modifier are only available when targeting ECMAScript 2015 and higher.

✅ 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

TypeScript disallows accessor in class fields when targeting ES5:

Properties with the 'accessor' modifier are only available when targeting ECMAScript 2015 and higher.

While I understand that there may be reason for TS to refuse transpiling accessor to ES5, it could allow using it in .d.ts files (or I guess in declare class classes).

📃 Motivating Example

babel/babel#17136

The .d.ts files for @babel/parser contain some accessor modifiers in classes. This shouldn't force consumers that want to transpile to ES5 to use skipLibCheck.

💻 Use Cases

Same as what I wrote in the motivating example.

[Andarist]

This is close - in its spirit - to #60438

[MartinJohns]

Also downlevel-dts, although this project seems to be dead? @sandersn