Union to Intersection

case1.ts

export type UnionToIntersection<U> = (U extends any ? (k: U) => void : never) extends (k: infer I) => void ? I : never

case2.ts

// @see https://stackoverflow.com/questions/50374908/transform-union-type-to-intersection-type

// union to intersection converter by @jcalz
// Intersect<{ a: 1 } | { b: 2 }> = { a: 1 } & { b: 2 }
type Intersect<T> = (T extends any ? ((x: T) => 0) : never) extends ((x: infer R) => 0) ? R : never

// get keys of tuple
// TupleKeys<[string, string, string]> = 0 | 1 | 2
type TupleKeys<T extends any[]> = Exclude<keyof T, keyof []>

// apply { foo: ... } to every type in tuple
// Foo<[1, 2]> = { 0: { foo: 1 }, 1: { foo: 2 } }
type Foo<T extends any[]> = {
    [K in TupleKeys<T>]: {foo: T[K]}
}

// get union of field types of an object (another answer by @jcalz again, I guess)
// Values<{ a: string, b: number }> = string | number
type Values<T> = T[keyof T]

// TS won't believe the result will always have a field "foo"
// so we have to check for it with a conditional first
type Unfoo<T> = T extends { foo: any } ? T["foo"] : never

// combine three helpers to get an intersection of all the item types
type IntersectItems<T extends any[]> = Unfoo<Intersect<Values<Foo<T>>>>

type Test = [
    { a: 1 } | { b: 2 },
    { c: 3 },
]

// this is what we wanted
type X = IntersectItems<Test> // { a: 1, c: 3 } | { b: 2, c: 3 }

// this is not what we wanted
type Y = Intersect<Test[number]> // { a: 1, b: 2, c: 3 }