Improve string split return type of first array index

[zardoy]

lib Update Request

Please, consider improving user experience for "".split(...) in cases when string literal is passed, which I believe most users do e.g. split(' ') or split('\n')

Configuration Check

My compilation target is ES2015 and my lib is the default.

Missing / Incorrect Definition

TypeScript/src/lib/es5.d.ts

Line 460 in b70784e

split(separator: string | RegExp, limit?: number): string[];

Essentially duplicate of #49635, but with #49635 (comment) handled:

split<T extends string | RegExp>(separator: T, limit?: number): T extends `${string}${infer U}` ? [string, ...string[]] : string[];

And this should added to param JSDoc then: Only passing `""` and `new RegExp("")` values can result in returning an empty array.

Sample Code

This is super inconvenient when used with destructure:

const [firstLine, ...restLine] = str.split('\n')
// firstLine is possibly undefined, need add exclamation marks everywhere or `as [string, ...string]` above

I also wanted to do the same for when regexp literal is passed e.g. split(/\n\r?/) as only split(new RegExp('')) can result in empty array (if I'm not mistaken), which I believe no one does, but the literal value can't be captured anyway

Documentation Link

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/split#description (note)

P.S. I remembered of this issue after #49682, which significantly improved type-checking experience by using less exclamation marks

[RyanCavanaugh]

FWIW: we have a pretty low appetite for making the standard library definitions extremely complex for the sake of NUIA, which is a flag we added with significant hesitation because it makes things like these difficult to handle.