Take a look at the code snippet below
interface Fn {
(val: never): void
}
const fn1: Fn = () => {}
const fn2: Fn = (val: number) => {}
const fn3: Fn = (val: { canBeAnyThing: string }) => {}
Despite the lack of errors, I find it puzzling. For further clarification, check out this live demo on TypeScript Playground
It seems I have discovered the solution.
First and foremost, we must understand the concepts of "covariance" and "contravariance"
Upon reviewing this article: What are covariance and contravariance?, we learn that
When it comes to contravariance in argument type, A ≼ B implies (B → T) ≼ (A → T) (where A and B switch positions).
Since never serves as the bottom type in TypeScript, it signifies
never is considered a subtype of every other type
(found here: When to use never and unknown in TypeScript)
By applying both rules, the code addressing the aforementioned question is perfectly valid.
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