Suggestion

Operator in can narrow the right-hand-side value, but it could also narrow the left-hand-side:

const a = {
    a: 1, b: 2, c: 3
} as const

const f = (char: keyof typeof a) => {}
const char = ['c', 'd'][Math.round(Math.random())]

if(char in a) {
    f(char)
}

char is 'c' | 'd' (or string), but if char in a is true, it should be able to narrow to 'c'.

🔍 Search Terms

type guard, narrow, operator in

✅ 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

As shown above: narrow also left-hand-side when possible.

📃 Motivating Example

As shown above

💻 Use Cases

As shown above