Allow annotations inside the head of a `for...in` statement

[todor2810]

Unlike element in for (let element of iterable) {} and error in try {} catch (error) {} where element and error are determined at run-time, key in for (let key in object) {} is already known to be a string at compile-time.

I propose we accept an annotation for key. In other words, the following syntax should be valid:

for (let key: string in object) {}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript / JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. new expression-level syntax)

[mhegazy]

string is not accurate here the correct type is Extract<keyof typeof obj, string>. Given that the possible types allowed here are limited, allowing an explicit type annotation seems more noise than value.

Please see #3500 for related discussion.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.