Proposal: Friend Declarations for classes

NOTE: This proposal is an alternative to the proposal in #5228.

Overview

Often there is a need to share information on types within a program or package that should not be
accessed from outside of the program or package. While the public accessibility modifier allows
types to share information, is insufficient for this case as consumers of the package have access
to the information. While the private accessibility modifier prevents consumers of the package
from accessing information from the type, it is insufficient for this case as types within the
package also cannot access the information. To satisfy this case, we propose the addition of a FriendDeclaration to classes:

class A {
  friend createA;
  private constructor() { }
}

function createA() {
  return new A();
}

A FriendDeclaration is a new declaration supported in the body of a ClassDeclaration or ClassExpression that indicates that the class, function, or namespace to which the supplied name resolves can treat the private and protected members of the class as if they were public.

The name referenced in the FriendDeclaration is resolved relative to the ClassDeclaration. The name must be in scope.

Grammar

  ClassElement: ( Modified )
   FriendDeclaration

  FriendDeclaration:
   friend FriendDeclarationNames ;

  FriendDeclarationNames:
   EntityName
   FriendDeclarationNames , EntityName

Examples

Instantiate a class with a private constructor:

class A {
  friend createA;
  private constructor() { }
}

function createA() {
  return new A();
}

Extend a class with a private constructor:

class A {
  friend B;
  private constructor() { }
}

class B extends A {
}

Access private members of an imported class:

// a.ts
import { update } from "./b";
export class A {
  friend update;
  private x: number;
}

// b.ts
import { A } from "./a";
export function update(a: A) {
  a.x = 1;
}

El issue no menciona archivos ni tests. Empieza comparando esta alternativa con la propuesta #5228 y validando la gramática y los ejemplos frente al diseño del lenguaje y del comprobador de tipos de TypeScript; se considera terminado cuando exista una implementación acordada de declaraciones friend que cubra el acceso private y protected en los casos mostrados.