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.

Empieza comparando la sintaxis de acceso solicitada y las restricciones de this con el patrón existente de métodos genéricos descrito en el issue. Investiga cómo informa actualmente TypeScript de An accessor cannot have type parameters.ts(1094) y cómo deberían comprobarse los ejemplos de accessors heredados. Se considera terminado cuando las declaraciones de accessors propuestas pasan la comprobación de tipos según lo previsto, los accesos heredados no válidos siguen produciendo errores y el comportamiento existente de salida de JavaScript no cambia.