Type is not assignable to type 'void' warning inconsistent

[jimmyfortinx]

TypeScript Version: 3.6.3

Search Terms:

Code

let returnVoid: () => void;
let returnUndefined: () => undefined;
const returnNumber: () => number = () => 3;

returnVoid = () => {
    return 2; // doesn't fail, but it should since number is not assignable to void
};

returnVoid = (): number => {
    return 2; // doesn't fail, but it should since number is not assignable to void
};

returnVoid = (): void => {
    return 2; // does fail because number is not assignable to void
};

returnVoid = () => {
    console.log('test')
}

returnUndefined = () => {
    return 2; // does fail, because number is not assignable to undefined
};

returnUndefined = () => {
    console.log('test') // does fail, because void is not assignable to undefined
}

returnVoid = returnNumber; // doesn't fail
returnUndefined = returnNumber; // does fail

Expected behavior:
() => number shouldn't be assignable to () => void

Actual behavior:
() => number is assignable to () => void

Goal:
We want to enforce people to return nothing from that function (also return; should be possible). Maybe I'm not using the right keyword, but I didn't found anything around that. Also, we don't want people to have to specify : void when they declare the function since nowhere else it's required because typing are correctly behaving.

Playground Link

Related Issues: It might be related to #33420, but the end goal is not the same

[j-oliveras]

Why are functions returning non-void assignable to function returning void?

[jimmyfortinx]

@j-oliveras Thanks for the link, I understand that use case, but would it be possible to have a new type like nothing (or whatever) that would make it possible to specify that we really want nothing?

[nmain]

But you already have that. If you explicitly annotate a void return on the function itself, Typescript does a stricter kind of check to handle exactly this use case, and any return <value> is errored.

[jimmyfortinx]

But that requires the developer to put :void everywhere he is creating a new function for that specific function type. I'll probably have to live with it then.