Narrowing for key type by `in` operator

Suggestion

A k in v check should narrow type of k: K to K & keyof typeof v.

🔍 Search Terms

in operator narrowing

✅ Viability 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. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

This code should compile due to narrowing on x:

const o = {foo: 1, bar: 2} as const;
const x: string = 'foo';
if (x in o) {
    const y: "foo" | "bar" = x; // OK
}

Currently it could be made with custom type guard

const in_ = <K extends string, O>(key: K, object: O): key is K & keyof O => key in object;
if (in_(x, o)) {
    const y: "foo" | "bar" = x; // OK
}

📃 Motivating Example

When validating a JSON with some kind of AST (or otherwise containing a tagged union) the only type we can have for a tag is a string.

const validateDisjointUnion = <O extends object>(
    nodeTypeValidators: { [K in keyof O]: (value: O[K]) => boolean },
    value: Json,
) => {
    if (typeof value !== 'object' || !('type' in value)) throw 42;
    const {type} = value;
    if (typeof type !== 'string' || !(type in nodeTypeValidators)) throw 42;
    // expression of type 'string' can't be used to index type '{ [K in keyof O]: (value: O[K]) => boolean; }'.
    return nodeTypeValidators[type](value);
}

💻 Use Cases

in_ solves the issue, but it involves a type guard. Type guards are "dangerous" in a sense that they can be used improperly (for example, !(key in object) would compile as well, but would lead to runtime error).

There is a related feature request to narrow type for second parameter of in operator. As far as I'm aware, there is no syntax to specify type of in_ that would guard on both parameters at the same time.

const guardBoth = <A, B>(a: A, b: B): (a is string) & (b is string) => { ... };

Edit. Not even

// A type predicate cannot reference a rest parameter.(1229)
const in_ = <K extends string, O, T>(...args: [K, O]): args is [K & keyof O, O & { [L in K]: unknown }] => args[0] in args[1];

No files or tests are named. Start with the TypeScript examples showing in checks and the custom type guard, then trace how the compiler narrows operands for the in operator. Done means the key narrows to K & keyof typeof v in the motivating cases without changing emitted JavaScript.