Object literals should have a `this` type too

TypeScript Version: 3.7.0-dev.20190831

Search Terms: this return type object composing composition

Code

I tried writing some self-contained parts of objects as a kind of multiple-inheritance or object composition, which I would combine later, and ran into this case.

const a = {
	Clone() {
		return this;
	},
	x: 1,
};

const b = {
	Clone: a.Clone,
	y: 2,
};

console.log(b.Clone().x); // number, bad! Should be an error!
console.log(b.Clone().y); // compiler error, bad! This should be acceptable!

Expected behavior: this, the return type of Clone, should evaluate to typeof b in b.Clone()

Actual behavior: It evaluates to typeof a, since it was defined in a

This can be circumvented by defining this as a type parameter like so:

const a = {
	Clone<T>(this: T) {
		return this;
	},
	x: 1,
}

const b = {
	Clone: a.Clone,
	y: 2
};

console.log(b.Clone().x); // compiler error, good!
console.log(b.Clone().y); // works! Also good!

Another interesting problem at play here though, is that using a type parameter for this also means that TS doesn't know what could be inside of this. This means you have to manually specify whatever members you hope to have access to. It would be nice if you could do T extends typeof a and it would automatically drop all the members of a in there. Right now, it will error if you do this:

'a' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer.ts(7022)

I think all methods like this should, therefore, be implicitly equivalent to the following:

const a = {
	Clone<T extends typeof a>(this: T) {
		console.log(this.x); // should work
		return this;
	},
	x: 1,
}

Then, b.Clone() would error since b is not a superset of a. Unless you did c = Object.assign({}, a, b), and did c.Clone()

Playground Link: https://www.typescriptlang.org/play/?target=6#code/MYewdgzgLgBAhjAvDA3gKAJAGEA24CmAFAJSqYYBO+UArhWDFABYCWEA3JgL4A0mAHgC4YARjRdOaUJFgAjJGWx4w+YXAB0uAnwwBPYQCZxk6RBA586vAHNCszcqLF1-YuxgB6DzBpgAJvgAZiwqflLgZhZWILb2Wiok6rpunt6gALYADiwWFDD4FBQgFEA

Related Issues: https://github.com/microsoft/TypeScript/issues/29122

リンク先の TypeScript Playground の例から始め、Clone を a から b にコピーしたときの推論される戻り値の型を比較してください。この動作の背景については、関連する issue #29122 を読んでください。完了条件は、オブジェクトリテラルのメソッドが、それを含むオブジェクトの使用から this を推論すべきかどうかを判断して実装し、同時に元のオブジェクトのメンバーへの有効なアクセスを維持することです。