Using an array union type A[] | B[] gets evaluated to A | B when accessing an element of the array

[jose-adalo]

Bug Report

Using an array union type A[] | B[] gets evaluated to A | B when accessing an element of the array

🔎 Search Terms

array union,
union of arrays,
accessing an element,
union of custom typed arrays,
accessing an array element

🕗 Version & Regression Information

V3.3.3 to
V4.9.4
(All typecript playground versions)

⏯ Playground Link

Playground link with relevant code

💻 Code

interface A {
      children:A[]
      a: number
}

interface B {
      children:B[]
      b: number
}

const func = (obj: A[] | B[]):void => {
      obj[0].children = [obj[1]]
      obj[0].children.push(obj[1])
}

🙁 Actual behavior

This gives a compilation error

Type '(A | B)[]' is not assignable to type 'A[] | B[]'.
  Type '(A | B)[]' is not assignable to type 'A[]'.
    Type 'A | B' is not assignable to type 'A'.
      Property 'a' is missing in type 'B' but required in type 'A'.(2322)

🙂 Expected behavior

No errors,
if obj is A[], then obj[1] is of type A and can be assigned to obj[0].children = [ obj[1] ],
else the same applied but with B

[RyanCavanaugh]

From TS's perspective, there's no difference between this code and

const func = (obj1: A[] | B[], obj2: A[] | B[]):void => {
      obj1[0].children.push(obj2[1])
}

which is clearly unsafe. We don't track where objects "came from" to establish higher-order relations.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[jose-adalo]

@RyanCavanaugh
I do agree that the second code, the one you provided, is unsafe and that is how it works now.

I know you don't track origin,
the only thing that's clearly confusing from a development experience is that
in my example:

obj: A[] | B[] can't have elements both A | B,
so when using two elements from itself, they should both have the same type.

As it is now A[] | B[] is similar to (A | B)[]

which is not what developers would expect.
Not even chatGPT lol

This can be a development choice due to the complications of higher order relations,
but is definetely an issue for developers.