Swift - setting dictionary value to nil confusion

/// If you assign `nil` as the value for the given key, the dictionary
/// removes that key and its associated value.

This is a sentence from the documentation, so as you can see it is by design. If this not fit your needs then you are doomed to use updateValue(value: Value, forKey: Hashable) which also works more predictable.

What I found is that when you use NSMutableDictionary instead of Dictionary then it works as "expected"

let ns = NSMutableDictionary(dictionary: d)
let x: Any? = nil
ns["foo"] = x // ns {}

However let x = Any? = nil case seems that there is a bug in Swift implementation at least to version Apple Swift version 3.0.1 (swiftlang-800.0.58.6 clang-800.0.42.1)

Btw. when all elements from Dictionary are removed, type of Dictionary is still correctly recognised

let x: String? = nil
d["foo"] = x // d is now [:]
let m = Mirror(reflecting: d) // Mirror for Dictionary<String, Optional<Any>>

I allowed myself to add bug to Swift lang: https://bugs.swift.org/browse/SR-3286

Given a dictionary like this:

dict: [String: String?] = []

If you add nil value like this:

dict["a"] = nil

The dictionary will be empty. However, if you add a nil value like this:

dict["a"] = nil as String?

The dictionary will not be empty. Try it out...

func testDictionary() {
    var dict: [String: String?] = [:]

    dict["abc"] = nil
    print("dictionary is empty -> \(dict.isEmpty)") // -> true

    dict["abc"] = nil as String?
    print("dictionary is empty -> \(dict.isEmpty)") // -> false
}

This seems very odd behavior and a subtle difference. Can someone explain?