InternalIsUnion

type InternalIsUnion<T, U = T> = (IsNever<T> extends true ? false : T extends any ? IsEqual<U, T> extends true ? false : true : never) extends infer Result // In some cases `Result` will return `false | true` which is `boolean`, // that means `T` has at least two types and it's a union type, // so we will return `true` instead of `boolean`. ? boolean extends Result ? true : Result : never

type InternalIsUnion<T, U = T> = (IsNever<T> extends true ? false : T extends any ? IsEqual<U, T> extends true ? false : true : never) extends infer Result // In some cases `Result` will return `false | true` which is `boolean`, // that means `T` has at least two types and it's a union type, // so we will return `true` instead of `boolean`. ? boolean extends Result ? true : Result : never