ChildUpdateFields

Helper for calculating the nested fields for a given type T1. This is needed to distribute union types such as undefined | {...} (happens for optional props) or {a: A} | {b: B}.

In this use case, V is used to distribute the union types of T[K] on Record, since T[K] is evaluated as an expression and not distributed.

See https://www.typescriptlang.org/docs/handbook/advanced-types.html#distributive-conditional-types