Nil checks not always working for Any in Swift

Question

I'm confused about how Swift checks for nil when I'm using the Any type.
Here's an example:

let testA: Any? = nil
let testB: Any = testA as Any
let testC: Any? = testB

if testA != nil {
    print(testA) // is not called as expected
}
if testB != nil {
    print(testB) // prints "nil"
}
if testC != nil {
    print(testC) // prints "Optional(nil)"
}

testA works as expected. The variable is nil, so the condition is false.

testB works not as expecte. The variable is nil, as shown by the print call. But the condition testB != nil evaluates to true. Why is this the case?

testC confuses me as well, since it is testC = testB = testA. So why should it behave differently than testA?

How would I need to write the if conditions if testB ... and if testC ... to not be true.
I'm looking for a solution that doesn't require me to know the type, like ...

if let testB = testB as String

Edit: I'm testing this with Swift 4 in an Xcode 9.1 Playground file.

Edit2:
Some information about the actual problem I want to solve. I'm getting a dictionary of type [String: Any?] that is created by a JSON parser. I want to check if the value for a given key is nil, but it doesn't work, when the key exists and the value is Optional(nil).

Example:

var dict = [String: Any?]()
var string = "test"
var optionalString: String?
dict["key1"] = string
dict["key2"] = optionalString

if dict["key2"] != nil {
    print(dict["key2"]) // should not be executed, but returns Optional(nil)
}

Answer 1

In Swift, nil is actually a concrete value(of type enum). testB which is of Any type, is holding the enum Optional with value none and hence the condition testB != nil is true.

This solves the mystery of how Any of testB is able to hold the nil value.

Coming to your actual problem, I tried this piece of code in Storyboard(Xcode 9.2) and it worked as expected.

var dict = [String: Any]()
var string = "test"
var optionalString: String?
dict["key1"] = string
dict["key2"] = optionalString

if let value = dict["key2"] {
    print(value) // doesn't get executed
}

For testB and testC, it seems like == check with nil should provide a solution but, because binary operand can not be used with two Any? operands, we can not use ==.

Using switch-case though is able to give correct results:

switch testB {
case Optional<Any>.none:
    print("value is none")
default:
    print("value is not none")
}

switch testC! {
case Optional<Any>.none:
    print("value is none")
default:
    print("value is not none")
}

O/P:
value is none
value is none