support narrowing types of function arguments based on overloads

[DetachHead]

Suggestion

🔍 Search Terms

overload 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

it would be nice if you could rely on function overloads to more easily narrow parameters

📃 Motivating Example

in this case, it's impossible for value2 to be undefined if value1 isn't

function foo(): void
function foo(value1: number, value2: number): void
function foo(value1?: number, value2?: number): void {
    if (typeof value1 !== 'undefined') {
        //Type 'number | undefined' is not assignable to type 'number'.
        // Type 'undefined' is not assignable to type 'number'.(2322)
        const bar: number = value2
    }
}

[fatcerberus]

I know there's at least one other issue discussing this, but I don't know what to search for to find it.

[MartinJohns]

Related: #28167

[MartinJohns]

Duplicate of #14515.

@fatcerberus: Just search for overloads in:title