(Typescript) Generic parameter doesn't compile correctly for conditional type when boolean is used

Question

I have the following type - when P is defined, the function should accept a parameter of type P and return value of type P. Otherwise, it should not take any parameters and 'return' void. For some reason it doesn't work for boolean and:

1. I don't understand why.
2. Is there any way to make it work for boolean without explicitly adding additional condition when P extends boolean (as that works).

export type Test<P = void> = P extends void
    ? () => void
    : (payload: P) => P

const test0: Test = () => {} // compiles

const test1: Test<number> = (payload: number): number => payload // compiles

const test2: Test<boolean> = (payload: boolean): boolean => payload // doesn't compile

Answer 1

First why:

boolean is distributed as union of true | false, so resulting type is
((payload: false) => false) | ((payload: true) => true)'.

More info here

---

How to make it work:

Only "naked types" are candidates for distribution. One of the possible ways to opt out:

type Test<P = void> = [P] extends [void] 
    ? () => void 
    : (payload: P) => P

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