Typescript WTF Moments 6: Non Collapsing Object Unions

Object unions of same type do not collapse

type a = 1 | 1 // 1, collapsed
//   ^?

type b = { a:1 } | { a:1 } // { a:1 } | { a:1 }, do not collapse
//   ^?

type c = { a:1 } | { a:1 } | { a:1 } // { a:1 } | { a:1 } | { a:1 }, do not collapse
//   ^?

playground

this should be fine most of the time but it can be problematic if we use identical check on it

union is equal to union, but not equal to non union

type o = 1 | 1 // 1
//   ^?

type p = { c:1 } | { c:1 } //  { c:1 } | { c:1 }, what?
//   ^?


type IsSame<T, U> = (<G>() => G extends T ? 1 : 2) extends <
    G
>() => G extends U ? 1 : 2
    ? true
    : false

type q = IsSame<p, {c:1 }> // false, huh?
//   ^?

type r = IsSame<p, {c:1 } | {c:1}> // true, ok
//   ^?

type s = IsSame<p, {c:1 } | {c:1} | {c:1}> // true, huh?
//   ^?

type k = IsSame<p, {c:1 } | {c:1} | {c:1} | {c:1}> // true, huh?
//   ^?

playground