Argument type should carry through to return value

[Arlen22]

I have a function that takes an input object, adds properties to it based on the arguments, and returns it. In this case, the argument is a string containing multiple key-value pairs, but if I know what those keys are, I can tell the type checker what type is going to be returned. But I don't feel like declaring the input type twice, since it be an object literal, as seen here.

declare function parseFields<S, T = {}>(text: string, result: T): typeof result & S;
//returns a type with two fields, not three
parseFields<{hello:string, world:string}>("hello=there&world=here", { hi: 'there' })

Instead of T = {}, I want to somehow tell it that it is supposed to infer it from result, yet still be able to specify S. I'm putting this in as a feature request, because I'm pretty sure there is no way to do this.

T = typeof result gives me the error that result cannot be found. Yet, I think that this might be the best way to imply this, although I don't know how that would fit into the overall flow. What about T = typeofarg result or T = typeof arg result?

[jcalz]

Workaround: use currying?

declare function parseFields<S>(): <T>(text: string, result: T) => S & T;
// returns value of type { hello: string; world: string; } & { hi: string; }
parseFields<{ hello: string, world: string }>()("hello=there&world=here", { hi: 'there' });

[DanielRosenwasser]

Looks like a duplicate of #16597 (sorry, I know this one was filed first but that's how it got triaged).

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.