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

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

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

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> {}