Support mixins for abstract classes

Search Terms

Related to #32122, but not the same.
abstract mixins classes

Suggestion

Right now we can't to extending abstract classes with mixins. So, it would be nice to create a solution for this case.

Use Cases

abstract class Test {
}

type Constructor = new (...args: any[]) => any;

function MyAwesomeMixin<T extends Constructor>(Constructor: Constructor) {
  abstract class MyAwesomeMixin extends Constructor {
  }
  return MyAwesomeMixin;
}

class A extends MyAwesomeMixin(Test) {
  /* Argument of type 'typeof Test' is not assignable to parameter of type 'Constructor'.
  Cannot assign an abstract constructor type to a non-abstract constructor type.(2345) */
}

Okay, try to create abstract constructor type:

type Constructor = abstract new (...args: any[]) => any;
/* Syntax error */

Hmm,..

Examples

Here is two way to create solution for this problem:

1. (simple) Allow create abstract constructor types

type Constructor = (new (...args: any[]) => any) | (abstract new (...args: any[]) => any);

2. (complex, but beautiful) Add new syntax to define mixins, and initially add support to extending abstract classes, like this:

abstract class Test {
   abstract mainMethod(): void;
}

mixin MyAwesomeMixin {
   public someMethod(): void {
      console.log('hello');
   }
}

class A extends MyAwesomeMixin(Test) {
   public mainMethod(): void {
      this.someMethod();
   }
}

// AND

abstract mixin MyAwesomeAbstractMixin {
    abstract someMethod(): void;
}

class B extends MyAwesomeAbstractMixin(Test) {
   public mainMethod(): void {
      this.someMethod();
   }
}

// Also this can add support to extending mixins without Mixin(Mixin2(Mixin3(Class)))

mixin MyAwesomeExtendedMixin extends MyAwesomeAbstractMixin {
   public someMethod(): void {
      ...
   }
}

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.

Issue にある abstract-class と mixin の例から始め、関連する issue #32122 を確認してください。提案されている constructor-type と mixin-syntax のアプローチを比較し、明記された TypeScript の設計上の制約を維持しながら abstract class の拡張をサポートする設計を判断してください。選択した動作が仕様として定義され、例で報告された constructor-type エラーが発生しなくなれば完了です。