Understanding type comparison of collection types

[tera]

@tera

Is that a good behaviour though (ability to cast an empty array to any array type)? What bad would it make if that cast was outlawed (and resulted in a runtime error)? It is trivial to make an empty array of the target type without a need of having an empty array of another type.. Plus it's somewhat dangerous and inconsistent – say, I wrote that cast and tested my app, but it so happens that during those tests the array was empty. And then after the app is shipped users start triggering the unhappy path of non empty arrays, causing runtime traps.

An illustrative example:

var array: [Int] = []
if Int.random(in: 0...1000) == 0 { // simulating some rare app condition
    array.append(42)
}
let strings = array as! [String] // all good most of the time
// crash once in a while

1. what bad would it make if we disallow such a cast?
2. if we were to disallow it could we do it easily or is it a too complicated change?
3. should we disallow it?

I'd say if there are no issues in (1) and we can do it (2) then we should do it (3).