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

Start with the linked Playground reproduction and compare the behavior discussed in issues 32080 and pull request 29571. Trace how the nested mixin calls infer the types of one and two; done means all six @ts-expect-error lines produce type errors, including the calls with extra arguments.