Flow sub/union typing

darn.js

// Union types work as subtypes
type Animal = { legs: number }
type Person = Animal & { name: string }

function numberOfLegs(a: Animal): number {
  return a.legs;
}

const p: Person = { legs: 2, name: "Adam" }
numberofLegs(p) === 2

// But a union of objects with the same key is not the same as an object with the union of those types
type Color = "green" | "red"
type AnimalHouse = { animal: Animal, color: Color }
type PersonHouse = AnimalHouse & { animal: Person }

function color(h: AnimalHouse): Color {
  return h.color;
}

color({color: "green", animal: p})
// Found error: Property 'name' not found in object type 'Animal'.
// This seems to show it's trying to make Person and Animal the same

// Naively, I would assume a simplification like this:
//  { a: B } & { a: C } => { a: B & C }
// Which then would type check?
// But I bet there's a good reason this isn't true.
// Can anyone enlighten me?

I think this is caused by Sealed object types.