[Generic type] Function type of the returned result is not mapped to the parameter

[inevermore]

🔎 Search Terms

"Generic Type","Function","Parameters","ReturnType"

🕗 Version & Regression Information

• This is a bug
• This changed between versions v5.6.0

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAKhDOwbmgXigbwFBSgQwC5MdcoBjAewBsKAnIxWgSwDsBzAbhIF8vcAjItlJR+dACYR6URq048uvLFkotE5PFSrwo6YRq1xEQkrhZ4AthCIByMpqpHgNgDSmoAMwCuLMkQA8MFAQAB7AECziOgDWECAUHrAISCgAfAAU0UQwAJS6qcQiuLQQwF60LJjc+DpOyJAA2jAAunyk3G64HVhKWKCQUADCDjp6JA0A0lCsULHxif0QCQbazSYi5lZEE2243r5Ei8v2WvCTzQ02+2Q2rQo9XFiSZFR4JZ4+ZMBMFJUnVABlYDMdhQQLBMIRKKzOLLYanFxQABiEPCkR08O0TQuV0+twyJAAjrQQERMWcWpdNhBbp0oAA6RlvNjwIgABTellKUng-iRqSwOSIACVSuUWPUIHyBcpVOoSvAvFRgLoVkCQWx0nYHE5XFAbHgbDVyL9EDkgA

💻 Code

type TestType = {
 a: {
 color: string;
 };
 b: {
 border: string;
 };
};

const calls = {
 callTest: {
 name: 'callTest',
 func: <T extends keyof TestType>(k: T) => {
 return {} as TestType[T];
 },
 },
};

type Calls = {
 [K in keyof typeof calls]: {
 name: K;
 func: typeof calls[K]['func'];
 };
};

declare const callString = <T extends keyof Calls, F extends Calls[T]['func']>(
 qry: Calls[T]['name'],
 ...args: Parameters<F>
): ReturnType<F>

const result = callString('callTest', 'a' as const)

🙁 Actual behavior

result type is {color: string} | {border: string}. The type of the returned result is not mapped to the parameter

• [ ]

🙂 Expected behavior

result type is {color: string}. The type should map to parameter 'a'

Additional information about the issue

No response

[jcalz]

I think for this to work TS would need to infer type arguments from indexed access types, but it doesn't. This isn't considered a bug but a missing feature, which is possibly implemented at #20126 and/or #53017, neither of which have been merged.

[inevermore]

Looking forward the feature

[RyanCavanaugh]

The feature needed here is the more complex "Given a set of parameter types, produce what an equivalent call expression would have done". ReturnType<F> is definitely not that.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.