Skip "uncallable" callback arguments when building union signatures (fix for #42558)

[Nathan Fenner (Nathan-Fenner)]

Adds a fix for the following case, where the orDefault function can't be used with empty arrays without adding type annotations:

functionorDefault<T,D>(x: T|null,d: D): T|D{if(x===null){returnd;}returnx;}constxs: string[]|null=["a","bc","def"];// y: string[] | never[]consty=orDefault(xs,[]);// ERR: TypeScript 4.1// Each member of the union type // (<U>(callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => U[])// | (<U>(callbackfn: (value: never, index: number, array: never[]) => U, thisArg?: any) => U[])// has signatures, but none of those signatures are compatible with each other. (2349)// ERR: TypeScript 4.2 (dev)// Parameter 'item' implicitly has an 'any' type.(7006)constyThen=y.map(item=>item.length);constyChain=orDefault(xs,[]).map(item=>item.length);

See #42558 for details/background.

[TypeScript Bot (typescript-bot)]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

[Nathan Shively-Sanders (sandersn)]

Nathan Fenner (@Nathan-Fenner) It seems like #42620 addresses the bug as well, and more comprehensively. Does you agree, Wesley Wigham (@weswigham)?

If that's the case, probably we should close this PR.

[Nathan Fenner (Nathan-Fenner)]

This test case from this PR might be helpful or interesting to #42620:

constf2: ((ask: (x: string,y?: number)=>void)=>void)|((ask: (x: number,y: never)=>void)=>void)=nullasany;f2((a,b)=>{a;// Ideally, should be stringb;// Ideally, should be number|undefined})

I think #42620 addresses other aspects, but I'm not 100% it handles this (rather uncommon) edge-case. Chucking it in could be useful to track its behavior in the future, anyway