How can one determine if a type is either void or any?
type IsAny = void extends any ? true : false // true
type IsVoid = any extends void ? true : false // true
Based on the above it's not possible to determine if a type is void or if it is any. Is there another way to determine which one of the two it is?
void and any are not mutually exclusive: a void is also an any.
You can see it by doing the union betweed them. If they were exclusive, void would not disappear to let only any:
type T = void | any // any
Knowing that, it is normal that your IsAny resolves to true.
As for your IsVoid type, it resolves to boolean, which is actually true | false. You can see that by changing true and false by something else like 1 and 2. You will then get 1 | 2.
This is because your any type could be either void or something else that is not void. So we don't know if it extends void or not.
To know if a type is exactly any and nothing more special, or exactly void and nothing less special, you can do like this (inspired from this answer):
type IsStrictAny<T> = 0 extends (1 & T) ? T : never;
type A = IsStrictAny<any> // any
type B = IsStrictAny<void> // never
type IsVoid<T> = T extends void ? T : never
type IsNotStrictAny<T> = T extends IsStrictAny<T> ? never : T
type IsStrictVoid<T> = IsVoid<T> & IsNotStrictAny<T>
type C = IsStrictVoid<any> // never
type D = IsStrictVoid<void> // void
