Why the swift compiler cannot use my subscript? - ios

Question

I have code that resembles this
I created custom subscript to do the unwrapping for me to make things easier.
enum MyEnum {
case one
case two
case three
}
struct MyStruct {
static var empty: Self {
return .init()
}
//Some Variables Here
}
class MyClass {
var index = 0
var data: [MyEnum: MyStruct] = [
.two: .empty,
.three: .empty,
.one: .empty
]
var allTypes: [MyEnum] {
switch Bool.random() {
case true:
return [.two, .three]
case false:
return [.one]
}
}
var currentSelectedType: MyEnum {
return allTypes[index]
}
func myMethod(type: MyEnum) {
let one: MyStruct = data[type]!
let two: MyStruct = data[currentSelectedType]!
let three: MyStruct = data[allTypes[index]]
let four: MyStruct = data[.one]
}
}
fileprivate extension Dictionary {
subscript(_ key: Key) -> Value where Key == MyEnum, Value == MyStruct {
get {
return self[key]!
}
set {
self[key] = newValue
}
}
}
in my MyClass myMethod why I have to forcely unwrapp one and two but not three and four otherwise I get a compile time error
let one: MyStruct = data[type] // Error Value of optional type 'MyStruct?' must be unwrapped to a value of type 'MyStruct'
let two: MyStruct = data[currentSelectedType] //Error Value of optional type 'MyStruct?' must be unwrapped to a value of type 'MyStruct'
Is there something I'm missing here?

Answer 1

I don't have an answer on why compiler isn't picking the expected overload in this situation.
I would recommend clarifying the overload you wish to use at call site, like following.
fileprivate extension Dictionary {
subscript(safe key: Key, defaultValue: Value = .empty) -> Value where Key == MyEnum, Value == MyStruct {
get { return self[key, default: defaultValue] }
set { self[key] = newValue }
}
}
With above, you can tell compiler explicitly to use your preferred overload.
func myMethod(type: MyEnum) {
let one: MyStruct = data[safe: type]
let two: MyStruct = data[safe: currentSelectedType]
let three: MyStruct = data[safe: allTypes[index]]
let four: MyStruct = data[safe: .one]
}