What does the following produce? type UnionToIntersection = (U extends any ? (x: U) => void : never) extends (x: infer I) => void ? I : never;

Question

TypeScript Associate — Hard

What does the following produce? type UnionToIntersection = (U extends any ? (x: U) => void : never) extends (x: infer I) => void ? I : never;

Accepted Answer

The code snippet defines a type UnionToIntersection that converts a union type to its intersection using conditional types and the infer keyword. By exploiting function parameter contravariance, the type system is able to infer the resulting intersection type.