extension Dictionary where Key == String {
    func value<T>(for key: Key, or: T) -> T {
        return self[key] as? T ?? or
    }
}

let json = ["x": "5", "b": "12"]

// ### Example 1 ###
// #################
// Returns 5, as expected, on all versions of Xcode
print(#line, NSDecimalNumber(string: json.value(for: "x", or: "0")))

// ### Example 2 ###
// #################
// Xcode 9.4.1 Swift 4.1: Prints 0.
// Xcode 10 GM Swift 4.2: Should be 0, Printing NaN.
// Xcode 10 GM Swift 4 mode: Should be 0, Printing NaN.
print(#line, NSDecimalNumber(string: json.value(for: "y", or: "0")))

// ### Example 3 ###
// #################
// Same, but explicitly casting, works correctly on all versions of Xcode
print(#line, NSDecimalNumber(string: json.value(for: "y", or: "0") as String))

// ### Example 4 ###
// #################
// Same, when extracting string separately.
// Works as expected on all versions of xcode
let value = json.value(for: "y", or: "0") // Returns "0" on all versions of Xcode
print(#line, NSDecimalNumber(string: value))