`Array` should extend `ReadonlyArray`

[DetachHead]

Bug Report

it's annoying how to extend arrays you have to duplicate your property in two interfaces

declare global {
    interface Array<T> {
      foo(): T
    }
    interface ReadonlyArray<T> {
      foo(): T
    }
}

this seems to be because Array only structurally extends ReadonlyArray, so adding to ReadonlyArray means Array no longer extends it

//true
type ArrayExtendsReadonlyArray = Array<unknown> extends ReadonlyArray<unknown> ? true : false

i believe the fix is to add this to Array

interface Array<T> extends ReadonlyArray<T> {}

🔎 Search Terms

array extends readonlyarray

🕗 Version & Regression Information

4.5.0-dev.20211015

⏯ Playground Link

Playground link with relevant code

💻 Code

interface ReadonlyArray<T> {
  foo(): T
}

//this returns true, but should return false because Array no longer extends ReadonlyArray
type ArrayExtendsReadonlyArray = Array<unknown> extends ReadonlyArray<unknown> ? true : false

declare const array: Array<unknown>

//error: Property 'foo' does not exist on type 'unknown[]'.(2339)
array.foo()

//but uncommenting this line will fix it:
// interface Array<T> extends ReadonlyArray<T> {}

🙁 Actual behavior

• have to add to both Array and ReadonlyArray, resulting in duplicate code
• Array<unknown> extends ReadonlyArray<unknown> ? true : false resolves to true even after adding members to ReadonlyArray that Array doesn't have

🙂 Expected behavior

Array should explicitly extend ReadonlyArray

[andrewbranch]

I think the best way forward is to open a draft PR so we can run all the extended tests on it and see if it breaks any projects in the wild.

[DetachHead]

Alright, I'll give it a go

[DetachHead]

#46781

[sandersn]

This is related to #36554 and should become part of that at some point.