Returned type of class decorator is ignored when class is decorated

[b-strauss]

Bug Report

Using decorators and proxies it's now possible to elegantly create classes which don't require the new keyword for instantiation. Unfortunately TypeScript doesn't allow this.

🔎 Search Terms

decorators, class decorators

🕗 Version & Regression Information

• Tested in 'Nightly' version

⏯ Playground Link

Playground link with relevant code

💻 Code

type Callable<T extends new (...args: any) => any> = T & ((...args: ConstructorParameters<T>) => InstanceType<T>);

function makeCallable<C extends new (...args: any) => any> (clazz: C): Callable<C> {
  return new Proxy(clazz, {
    apply (target, thisArg, argArray) {
      return new target(...argArray);
    },
  }) as unknown as Callable<C>;
}

function callable<C extends new (...args: any) => any> (clazz: C, context: ClassDecoratorContext<C>): Callable<C> {
  return makeCallable(clazz);
}

// Using function
const User = makeCallable(class {
  constructor (name: string, age?: number) {}
});

const x = new User('foo');
console.log(x instanceof User);

const y = User('bar'); // works
console.log(y instanceof User);

// Using decorator
@callable
class DecoratedUser {
  constructor (name: string, age?: number) {}
}

const a = new DecoratedUser('foo', 19);
console.log(a instanceof DecoratedUser);

const b = DecoratedUser('bar', 12); // Value of type 'typeof DecoratedUser' is not callable.
console.log(b instanceof DecoratedUser);

🙁 Actual behavior

TypeScript does not allow for the proxied class to be called, although this is valid js code and all instanceof checks are passing.

🙂 Expected behavior

TypeScript should use the return type of the decorator as the new type for the class.

[jcalz]

This is probably less "bug" and more "missing feature". Cross-linking to #51347

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.