Includes

The solution

type Includes<T extends readonly unknown[], U> =
  T extends readonly [infer H, ...infer R]
    ? Equal<H, U> extends true
      ? true
      : Includes<R, U>
    : false

The common wrong answer

type Includes<T extends readonly unknown[], U> =
  U extends T[number] ? true : false

Short and wrong. `T[number]` is the union of element types and `extends` asks about assignability, so `Includes<[boolean], true>` answers `true` — `true` is assignable to `boolean` even though the tuple contains no such element. Assignability is not membership.

Line by line

1. Equal<H, U> extends true

   `Equal` returns the type `true` or the type `false`, so it has to be tested with `extends true` rather than used directly as a condition. This is the puzzle where the harness itself becomes a tool.

2. : false

   Reaching the empty tuple means every element was checked and none matched. Exhausting the list *is* the negative answer.

Takeaway

When a puzzle says "exactly", `extends` is the wrong tool — it tests assignability, which is one-directional. Exact identity needs `Equal`, and this is why every check on this site is built on it.