Convert members of a union to an intersection.

UnionToIntersection.ts

/**
 * Convert members of a union to an intersection.
 *
 * @example
 * ```ts
 * type Union = { a: number } | { b: string };
 * type Intersection = UnionToIntersection<Union>;
 * // { a: number } & { b: string }
 * ```
 *
 * @privateRemarks
 * This works by taking advantage of [distributive conditional
 * types](https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#distributive-conditional-types),
 * which allows conditional types to be applied to each member of a union type
 * individually, and [contravarience in function argument
 * types](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-6.html#strict-function-types).
 *
 * The conditional type `T extends any ? (x: T) => any : never` is used to
 * create a function type for each member of the union that takes the member as
 * an argument.
 *
 * Then, the union of function types is checked to see if it can be assigned to
 * a single function type with an inferred argument type. TypeScript infers the
 * argument type as the intersection of the union members since it's the only
 * argument type that satisfies all members of the function type union.
 */
type UnionToIntersection<T> = (
  T extends any ? (member: T) => any : never
) extends (member: infer R) => any
  ? R
  : never;