Subscript of a struct doesn't set values when created as an implicitly unwrapped optional - swift

Question

Why can't I change the the "numbers" array using subscripts when "Foo" is an implicitly unwrapped optional?
struct Foo {
var numbers = [0,0,0]
subscript(index: Int) -> Int {
get { return self.numbers[index] }
set { self.numbers[index] = newValue }
}
}
var fooA:Foo!
fooA = Foo()
fooA[1] = 1 // does not change numbers array
fooA[1] // returns 0
fooA.numbers[1] = 1 // this works
fooA[1] // returns 1
var fooB:Foo!
fooB = Foo()
fooB![1] = 1 // this works
fooB![1] // returns 1
For some reason it works when I make "Foo" a class (called "Goo" below)
class Goo {
var numbers = [0,0,0]
subscript(index: Int) -> Int {
get { return self.numbers[index] }
set { self.numbers[index] = newValue }
}
}
var goo:Goo!
goo = Goo()
goo[1] = 1 // this works
goo[1] // returns 1

Answer 1

it looks like a bug (or i miss something important), check this
struct Foo {
var numbers = [0,0,0]
subscript(index: Int) -> Int {
get {
return self.numbers[index]
}
set {
numbers[index] = newValue
}
}
}
var fooA:Foo! = Foo()
// here is the difference
fooA?[1] = 1
fooA[1] // 1
fooA.numbers[1] = 1
fooA[1] // 1
more 'complex' experiment
struct Foo {
var numbers = [0,0,0]
subscript(index: Int) -> Int {
get {
return numbers[index]
}
set {
print(numbers[index],newValue)
numbers[index] = newValue
print(numbers[index])
}
}
}
var fooA:Foo! = Foo()
fooA[1] = 1
fooA[1] // 0
// but prints
// 0 1
// 1
for more 'fun'
var fooA:Foo! = Foo()
if var foo = fooA {
foo[1] = 1
print(foo)
}
prints
"Foo(numbers: [0, 1, 0])\n"