Open Radar

 
As documented in the Swift Programming Guide, Array is supposed to act like a value type, except when it doesn't. This adds a weird corner case to the language and will no doubt prove a source of all kinds of wonderful bugs.

Currently this is how it works:

let a = [1, 2, 3]
var b = a
b[1] = 5
a  // [1, 5, 3]

b.append(4)
a  // [1, 5, 3]
b  // [1, 5, 3, 4]

The array is not copied unless its length changes. 

This behavior also exists when the array is part of another value type:

struct Yummie {
  var a = [1, 2, 3]
}

var x = Yummie()
var y = x
y.a[1] = 5
x.a  // [1, 5, 3]

This behavior is really hard to reason about. 

It is more reasonable to assume that doing var b = a makes a real copy. The unshare() function should not need to exist, and unsharing should happen automatically behind the scenes when an array is placed into a new variable.

The current behavior of Array seems to have been designed only with runtime efficiency in mind, not with how human beings think.