Repository: TypeScript
Issue: Mixin constructor not working with `args:unknown[]`, only with `args:any[]`
Labels: Suggestion,Awaiting More Feedback

### 🔎 Search Terms

[site:github.com/microsoft/typescript mixin class "unknown" args any args](https://www.google.com/search?num=10&sca_esv=34e05643fe6f2863&q=site:github.com/microsoft/typescript+mixin+class+%22unknown%22+args+any+args&sa=X&ved=2ahUKEwiRqLuRxpGKAxW_HzQIHV_0K0kQ5t4CegQIKxAB&biw=1911&bih=1094&dpr=1.75)

### 🕗 Version & Regression Information

- **This changed most likely after `unknown` was introduces and the bespoke/unique mixin type checking logic wasn't updated**.

### ⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAwg9gOwM7AE4FcDGw6qgXigQgHcoAKAOmoENUBzJALinQQGsE4SEBtAXQCUBAHxQ4AIwBWEbACg5AMzbYAloigAxOHAA8AFSgQAHsAgIAJkliIUGbLhHkAQjSQQW+4QG85Uf1CoEMDoqAhQmAA2btbacEam5lZQru5QvgGZEbZoWDioVLQMzKwcXDwCPn5ZNUjokAXUlHSMgtU1AL4KNf6KOgRQAIwATADM3T19cACywQAWcBbkPoHBoeHAc6pIlFNQXZldXUA

### 💻 Code

```ts
type Constructor = new (...args: unknown[]) => object

function Foo<T extends Constructor>(Base: T) {
    return class Foo extends Base { // <---------------- ERROR
        constructor(...args: unknown[]) {
            super(...args)
        }

        foo = 123

        fooMethod() { return this.foo }
    }
}
```

Change the `unknown` to `any` and the error goes away. You'd think they should behave the same: you don't care what the args are, you just need to pass them along.

Here's how to fix the error, changing `unknown`s to `any`s:

[playground](https://www.typescriptlang.org/play/?#code/C4TwDgpgBAwg9gOwM7AE4FcDGw6qgXigQgHcoAKAOmoENUBzJALihoRAG0BdASgID4ocAEYArCNgBQkgGboE2AJaIoAMThwAPABUoEAB7AICACZJYiFBmy5+5AEI0kEFtr4BvSVG9RUEYOioCFCYADZO5upweobGZlCOzlCePqkhlmhYOKhUtAzMrOzcHl5pZUjokDnUlHSMPKVlAL7SZd4yGgRQAIwATADMrW0dcACy-gAWcCbkHr7+gcHAE4pIlCNQLaktLUA)

Code:

```ts
type Constructor = new (...args: any[]) => object

function Foo<T extends Constructor>(Base: T) {
    return class Foo extends Base { // <---------------- ERROR
        constructor(...args: any[]) {
            super(...args)
        }

        foo = 123

        fooMethod() { return this.foo }
    }
}
```

### 🙁 Actual behavior

Type error on `Foo`.

### 🙂 Expected behavior

I expected it to allow "any unknown args" to be passed along.

### Additional information about the issue

Plus, with an `unknown` type, the code in the mixin `constructor` can actually be more type safe, requiring type narrowing or type casting to use the args at all, whereas `any` allows anything to be done to the args which is not safe.

Maybe the mixin type checking algo could cast the base class constructor to `new (...args: any[])` and simply allow anything to be passed in, to make `unknown` work. Then the mixin type checking algo could still enforce `any[]` as well as (more preferably) `unknown[]`.
 
Also, maybe it would make sense to have an implicit `Constructor` type built into the mixin type checking algo so that mixins can just be really easy for plain JS users who are migrating to TypeScript.

I've met multiple people who are baffled by how difficult it is to write mixins in TypeScript when they come from plain JavaScript.