Types for async functions do not handle `void` assignments like normal ones

TypeScript Version: 3.4.0-dev.201xxxxx

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Code

const w: void = "";

const x: () => void = () => "";

const y: () => Promise<void> = async () => "";

function a(): void {
    return "";
}

async function b(): Promise<void> {
    return "";
}

const y2: () => Promise<() => void> =  async () => () => "";

function a2(): () => void {
    return () => "";
}

function b2(): Promise<() => void> {
    return () => "";
}

Expected behavior: Generally speaking, all of them should be an error, or all of them shouldn’t be.

Actual behavior: We have a special “function returning void” rule that allows () => "" to be assignable to () => void, but this doesn’t carry thru to async scenarios, so a async () => "" is not assignable to a () => Promise<void>.

Playground Link

[weswigham]

Tbh, I think void just needs to behave like unknown except we also have lint-level checks to attempt to forbid real-valued returns in implementations whose expected return position type is void.

[fatcerberus]

I don’t know, I think it’s kind of elegant right now that void is in a duality with any. 😃

• any: Do whatever you want, I won’t bother you but runtime errors are your problem now
• void: I’ll put whatever here, don’t try to stop me (by inspecting the value) or else

Where “you” = programmer, “I” = type system.