Option to make `this` in functions inside object literals typed

[FranklinWhale]

The lack of IntelliSense to this in functions inside object literals was reported in #7801 and #8110. #8382 attempted to fix that but it was reverted via #8389 due to #8191.

Adding this: this to functions in an interface, as mentioned in #9583, was a suggested way to workaround this issue. However, it is not convenient to define this: this for each method of an interface. Also, even when the object implements an interface, it may contain additional properties that are used in the interface methods but not exposed via the interface. Finally, like microsoft/vscode#298, there may be no interface at all.

Although typing this in functions inside object literals cannot be switched on by default, I think there should be a way to opt in. Could something be applied to an object literal to explicitly mark this in all methods of that object be the type of the surrounding object?

In the following examples, @ is a symbol that

1. When it is placed immediately before the open bracket of an object literal, this in all methods of that object is of the type of the surrounding object.
2. When it is located after this: in a method declaration inside an object literal, this in that method is of the type of the surrounding object.

Example 1--this being the surrounding object in all methods of an object literal:

let o = @{
	prop: 12,
	bar() {
		this.prop = 3; // `this` is inferred
	},
	foo() {
		this.bar(); // `this` is inferred
	}
};

Example 2--this being the surrounding object in a method of an object literal:

let o = {
	prop: 12,
	bar(this: @) {
		this.prop = 3; // `this` is inferred
	},
	foo() {
		this.bar(); // `this` is any here
	}
};

Example 3--this being the surrounding object in all methods of an object literal that implements other interfaces:

interface I {
	bar(); //`this: this` not required
	foo(); //`this: this` not required
}

let o1: I = @{
	prop: 12,
	bar() {
		this.prop = 3; // `this` is inferred
	},
	foo(this: any) {
		this.bar(); // `this` is any
	}
} as I;

let o2: I = <I>@{
	prop: 12,
	bar() {
		this.prop = 3; // `this` is inferred
	},
	foo(this: any) {
		this.bar(); // `this` is any
	}
};

[RyanCavanaugh]

var o: this already means something else, so you'd need to find some other syntactic place to put this.

[mhegazy]

We have tried changing this behavior a while back and ran into some interesting scenarios that would not work, some relevant comments in #7801 (comment)

[FranklinWhale]

@RyanCavanaugh: I have updated the examples to use @ instead of : this. I think that should work.

@mhegazy: I have read the comments and understand that this being any should be the best default. However, I think there should be a way to opt-in the behavior in #8382. What do you think about the use of the @ symbol?

[gdh1995]

I think it's badly needed!

It's too terrible to create an interface for every Object literal!

And I hate that I must declare a new member function whenever I want to add it to the origin literal.

As for POJO, well, this: Ember is much easier to write.

Anyone has a forked version of TypeScript 2 and the linter which support "auto-typed" this?

[ahejlsberg]

See #14141.

[FranklinWhale]

I think the suggested feature is no longer necessary.