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Using ReturnType with generic function parameters and generic property access #26856
Description
TypeScript Version: 3.1.0-dev.20180901
Search Terms: ReturnType, generic, invoke, invoker, property
Code
function invoker <K extends string | number | symbol, A extends any[]> (key: K, ...args: A) {
return <T extends Record<K, (...args: A) => any>> (obj: T): ReturnType<T[K]> => obj[key](...args)
}
const result = invoker('test', true)({ test: (a: boolean) => 123 })Workaround (duplicate similar ReturnType-like type locally):
export type InvokeResult <T extends (...args: A) => any, A extends any[]> = T extends (...args: A) => infer R ? R : never
export function invoker <K extends string | number | symbol, A extends any[]> (key: K, ...args: A) {
return <T extends Record<K, (...args: A) => any>> (obj: T): InvokeResult<T[K], A> => obj[key](...args)
}Expected behavior: Valid program and use of ReturnType.
Actual behavior:
Type '(...args: A) => any' does not satisfy the constraint '(...args: any[]) => any'.
Types of parameters 'args' and 'args' are incompatible.
Type 'any[]' is not assignable to type 'A'.
Related Issues: No, this is out of my skillset to debug tonight. Logging for posterity and help 😄 I did try different workarounds and found I couldn't workaround it with any usage of property access so I added that to the title. For instance, the exact same code inlined results in an empty interface:
export function invoker <K extends string | number | symbol, A extends any[]> (key: K, ...args: A) {
return <T extends Record<K, (...args: A) => any>> (obj: T): T[K] extends (...args: A) => infer R ? R : never => obj[key](...args)
}
const result = invoker('test', true)({ test: (abc: boolean) => 123 })More minimal repro which makes me realise it may not be an issue but maybe an enhance to ReturnType to specify the generic type parameters:
type Test <T extends (...args: A) => any, A extends any[]> = ReturnType<T>
Activity
ahejlsberg commentedon Sep 3, 2018
Yeah, that error is unfortunate. Here's a minimal repro:
In
--strictFunctionTypesmode this produces an error similar to the one you're seeing, the issue being that(...args: A) => stringdoes not satisfy the constraint(...args: any[]) => anybecauseany[]is not assignable toA. It seems that we need to be a little more permissive in the way we check assignability of rest parameters with typeany[].Voronar commentedon Jan 18, 2019
Less safety version of
ReturnTypebut working for all inputs:Update1
But inference not working properly for all cases.
Cases above working correct.
But not working for union type rest parameters case (with enabled
strictFunctionTypes):