Methods in simple mixin result as type any.

TypeScript Version: 3.9.2

Search Terms:

typescript mixin method is any

Code

type AnyCtor = new (...a: any[]) => any

function Foo<T extends AnyCtor>(Base: T) {
	return class Foo extends Base {
		foo() {}
	}
}
function Bar<T extends AnyCtor>(Base: T) {
	return class Bar extends Base {
		bar() {}
	}
}
function One<T extends AnyCtor>(Base: T) {
	return class One extends Base {
		one() {}
	}
}
function Two<T extends AnyCtor>(Base: T) {
	return class Two extends Base {
		two() {}
	}
}
function Three<T extends AnyCtor>(Base: T) {
	return class Three extends One(Two(Base)) {
		three() {}
	}
}

class MyClass extends Three(Foo(Bar(Object))) {
	test() {
		// @ts-expect-error
		this.foo(123)
		// @ts-expect-error
		this.bar(123)
		// @ts-expect-error // ERROR
		this.one(123) // this.one is type `any`!
		// @ts-expect-error // ERROR
		this.two(123) // this.two is type `any`!
		// @ts-expect-error
		this.three(123)
		console.log('no runtime errors')
	}
}

const m = new MyClass()
m.test()

Expected behavior:

There should be an error on all the lines marked with // @ts-expect-error

Actual behavior:

There is no type error on the lines with this.one and this.two because this.one and this.two are seen as type any.

The expectation is that the one and two methods have the proper type (with zero parameters) which would therefore cause a type error from passing in arguments.

Playground Link

Related Issues:

Someone from the Discord chat thought perhaps https://github.com/microsoft/TypeScript/pull/29571 might be related, but not with 100% certainty. They do however think this is a bug.

Ultimately, making mixins in TypeScript is too hard. It's too easy to get them wrong and they become a very inconvenient in TypeScript (whereas they are very convenient in plain JavaScript).

Also related: https://github.com/microsoft/TypeScript/issues/32080

リンクされている Playground の再現から始め、issue 32080 と pull request 29571 で説明されている動作を比較します。ネストされた mixin 呼び出しが one と two の型をどのように推論するかを追跡します。6 行すべての @ts-expect-error 行が型エラーを生成し、追加の引数を指定した呼び出しも含まれる状態が完了条件です。