Allow narrowing of unions discriminated by numeric literals using `>` `<` etc

🔎 Search Terms

"arguments length"

🕗 Version & Regression Information

It works the same in v3.3.3

⏯ Playground Link

https://www.typescriptlang.org/play/?target=99&module=0&ts=5.8.3#code/GYVwdgxgLglg9mABAUwB4EMC2AHANsgRgAoA6M9AJwHMBnALkQG0wRMAjZCgXUQB8muASkQBvAFCJJiGMERFKtEvjBUoAC0QA+RAAZh4qYcQVkUEBSQKajHVwDcEqQF8UuGslGOjx0+aQA5dH8HQycxMNBIWAQUDBx8ACZScmp6JhZ2Th5+RiFPQxk5KyVkFXV9LyMTMwtEKxt7SpdkNw8Dbx8agKCQ53CxSOh4JDQsPGQAZmSSKwZmVg5uPgEKgtl5VJKyjQBePd1Vjuq-REDgptd3fKPfWvrbXskwpyA

💻 Code

Some basic ways to check if we have any arguments could be these:

function example1(...args: [number] | []) {
    if (args.length > 0) {
        return args[0];
    } else {
        return NaN;
    }
}
function example2(...args: [number] | []) {
    if (args.length) {
        return args[0];
    } else {
        return NaN;
    }
}
function example3(...args: [number] | []) {
    if (args.length === 0) {
        return NaN;
    } else {
        return args[0];
    }
}

But TS doesn't seem to understand the first one.

🙁 Actual behavior

It doesn't understand the way it's done in "example1".

🙂 Expected behavior

It should understand the way it's done in "example1".

Additional information about the issue

Type inference on "args" after the check for "example1":

Type inference on "args" after the check for "example2":

Type inference on "args" after the check for "example3":

Notice how for "example2" and "example3" the type is correctly narrowed to just [number], but for "example1" the unnarowed [number] | [] type remains.

Use the linked TypeScript Playground examples as the entry point; compare control-flow narrowing for args.length > 0 with the existing truthiness and equality cases. Done means example1 narrows [number] | [] to [number] in the true branch without losing the behavior shown by examples 2 and 3.