Allow accessors to support inline generics for this

Search Terms

getter, accessor, this, generic

Suggestion

Currently accessor declarations do not support generics at the declaration site. The error returned is An accessor cannot have type parameters.ts(1094). Currently the accessor can implement generic declaration via the enclosing class. The goal would be to allow for the accessor to support a generic this at the declaration site.

Use Cases

The use case is to allow for more expressive setter/getter declarations, in line with how method declarations currently work.

Examples

Suggested Pattern

Below is an example of defining the constraints of this at the accessor site, to determine if an inherited accessor method is valid by the shape of the subclass.

class Base {
   get left<T extends { _left: string}>(this: T) {
       return this._left;
   } 
   get right<T extends { _right: string}>(this: T) {
       return this._right;
   } 
}

...
class Left extends Base {
    _left: string = '';
}
new Left().left; // OK
new Left().right;  // Errors

...
class Right extends Base {
   _right: string = '';
}
new Right().right; // OK
new Right().left; // Errors

...
class Both extends Base {
   _right: string = '';
   _left: string = '';
}
new Both().right; // OK
new Both().left; // OK

This pattern can currently be emulated by converting the accessors into standard methods

class Base {
  getLeft<T extends { _left: string}>(this: T) {
      return this._left;
  } 
  getRight<T extends { _right: string}>(this: T) {
      return this._right;
  } 
}

...
class Left extends Base {
   _left: string = '';
}
new Left().getLeft(); // OK
new Left().getRight(); // Errors

...
class Right extends Base {
  _right: string = '';
}
new Right().getRight(); // OK
new Right().getLeft(); // Errors

...
class Both extends Base {
  _right: string = '';
  _left: string = '';
}
new Both().getRight(); // OK
new Both().getLeft(); // OK

Checklist

My suggestion meets these guidelines:

• 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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

まず、要求されたアクセサー構文と this の制約を、issue で説明されている既存のジェネリックメソッドパターンと比較します。TypeScript が現在 An accessor cannot have type parameters.ts(1094) をどのように報告するか、また継承されたアクセサーの例をどのようにチェックすべきかを調査します。提案されたアクセサー宣言が意図どおり型チェックされ、無効な継承アクセサーへのアクセスが引き続きエラーになり、既存の JavaScript 出力の動作が変わらなければ完了です。