The standard library signature for Array<T>.includes(u) assumes that the value to be checked is of the same or narrower type than the array’s elements T. But in your case you are doing the opposite, checking against a value which is of a wider type. In fact, the only time you would say that Array<T>.includes<U>(x: U) … Read more
You’ll need to use a mapped tuple type, which is supported in TypeScript 3.1. You can make a mapped type that has properties 0, 1, 2 and length of the correct types, like this: class Maybe<T> {} type MaybeTuple = [Maybe<string>, Maybe<number>, Maybe<boolean>]; type MaybeType<T> = T extends Maybe<infer MaybeType> ? MaybeType : never; type … Read more
You can read more about unknown in the PR or the RC announcement, but the gist of it is: [..] unknown which is the type-safe counterpart of any. Anything is assignable to unknown, but unknown isn’t assignable to anything but itself and any without a type assertion or a control flow based narrowing. Likewise, no … Read more