Array of intersection is not intersection of arrays

[jsalvata]

🔎 Search Terms

array intersection [] & distributive law

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Type System Behaviour.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.2#code/C4TwDgpgBAggTnAhiA8gMwJIDtgTgZwgGNgBLAeyygF5YFkAeAbygA8AuKLAVwFsAjPFAC+UAGRQWITvmBxSWAOYiAfAG4AsAChQkKNlwFiZSunhIQ+GnQvM2nHgKHCV4m4yky5C5S81btABsIYChEclJOc2R0AzxCEgosf2DQikROOKNE0zRoy39tNG4sHKpcWQAKAEpJbSgodOtw0n8Glut0-2FtIA

💻 Code

type ArrayOfIntersection = Array<{ x: number } & { y: string }>;
type IntersectionOfArrays = Array<{ x: number }> & Array<{ y: string }>;

let aoi: ArrayOfIntersection;
let ioa: IntersectionOfArrays;

function test() {
  ioa = aoi;
  aoi = ioa; // Type error
}

🙁 Actual behavior

The aoi = ioa assignement has a type error, while Array<A & B> should be exactly equal to Array<A> & Array<B>:

• Values in an Array<A & B> have type A & B.
• Values in an Array<A> & Array<B> have type A and also have type B -- that is: they have type A & B.
  So these two types are equivalent, yet Typescript accepts assignment in one direction but not the other.

🙂 Expected behavior

The above code should compile without error.

Additional information about the issue

No response

[jcalz]

Duplicate of or strongly related to #41874 and the issues linked within. Not sure if you actually searched for issues with search terms like array intersection in:title and if you think this issue is distinct from those, or if you just didn't find them when you searched. Intersections of arrays in TypeScript are known to be weird and it's considered a design limitation.

[typescript-bot]

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