What does the following type compute and what technique does it use? type UnionToIntersection = (U extends any ? (x: U) => void : never) extends (x: infer I) => void ? I : never;

Question

TypeScript Professional — Hard

What does the following type compute and what technique does it use? type UnionToIntersection = (U extends any ? (x: U) => void : never) extends (x: infer I) => void ? I : never;

Accepted Answer

The type UnionToIntersection converts a union to an intersection by leveraging contravariance of function parameters. Each union member is transformed into a function parameter type that must satisfy all simultaneously through inference.