When trying to use mapped tuples as rest parameters error 'A rest parameter must be of an array type' given

TypeScript Version: 3.2

Search Terms: mapped tuples rest

Code

type FuncParams<T> = T extends (...args: infer P) => any ? P : never;
type Stringify<T> = {
    [K in keyof T]: string;
};
type Optional<T> = {
    [K in keyof T]?: T[K];
};

type ThreeParamFunc = (paramOne: string, paramTwo: number, paramThree: boolean) => void;

type Params = FuncParams<ThreeParamFunc>; // [string, number, boolean]
type StringParams = Stringify<FuncParams<ThreeParamFunc>>; // [string, string, string]
type OptionalParams = Optional<FuncParams<ThreeParamFunc>>; // [string?, number?, boolean?]

function doStuff<T>(func: T, ...params: FuncParams<T>) { // works fine
}

function doOptionalStuff<T>(func: T, ...params: Optional<FuncParams<T>>) { // A rest parameter must be of an array type.
}

function doStringStuff<T>(func: T, ...params: Stringify<FuncParams<T>>) { // A rest parameter must be of an array type.
}

Expected behavior: I should be able to use a mapped tuple as a rest param in a function

Actual behavior: I get the error A rest parameter must be of an array type.

Playground Link: Link

[Roaders]

I’d be interested to hear why this is marked as a suggestion rather than a bug. As far as I see it you can usually use a tuple type as a function parameter but in this scenario (and other similarly complicated scenarios) it doesn’t work.

Can anyone explain why it doesn’t work in this case? I have tested my mapped type with function parameters and on it’s own it works fine as a rest param:

type FunctionParams<T> = T extends (...params: infer R) => any ? R : never;

type matchFunc<T> = (value: T) => boolean;

interface IMatcher<T>{
    match: matchFunc<T>;
    toString: () => string;
}

type MatchUnion<T> = matchFunc<T> | IMatcher<T>;

type ToMatchers<T extends any[]> = {
    [P in keyof T]: MatchUnion<T[P]>;
}

interface ISample{
    funcOne(one: string, two: number, three: boolean): true;
}

const sample: ISample = {} as any;

type MappedParams= ToMatchers<FunctionParams<ISample["funcOne"]>>;

function processMatchers(...params: MappedParams) {
}

playground

[Andarist]

Having the same problem - prepared a simpler repro (with artificial code ofc).

Real world use case would be to cover reselect’s createSelector API in generic manner - https://github.com/reduxjs/reselect#createselectorinputselectors--inputselectors-resultfunc