Is it a union?

The solution

type IsUnion<T, Whole = T> =
  [T] extends [never]
    ? false
    : T extends Whole
      ? [Whole] extends [T] ? false : true
      : never

The common wrong answer

type IsUnion<T, Whole = T> = T extends Whole ? [Whole] extends [T] ? false : true : never

It handles ordinary types and unions, but a naked conditional over never runs zero times and returns never. The explicit wrapped guard is the only branch that gets a chance to call it false.

Line by line

1. [Whole] extends [T]

   Inside a distributed branch T is one member while Whole still holds the original union. The wrapped comparison detects that difference without distributing again.

Takeaway

Union detection works by comparing one distributed member with a preserved whole; never needs its own guard because it produces no members.