How to solve “String interpolation produces a debug description for an optional value; did you mean to make this explicit?” in Xcode 8.3 beta?

This is a change that was made in this pull request due to the fact that interpolating Optional(...) into the resultant string is often undesirable, and can be especially surprising in cases with implicitly unwrapped optionals. You can see the full discussion of this change on the mailing list here.

As mentioned in the pull request discussion (although unfortunately not by Xcode) – one slightly nicer way to silence the warning than the use of String(describing:) is to add a cast to the optional type of whatever you’re interpolating, so for example:

var i: Int? = 5
var d: Double? = nil

print("description of i: \(i as Int?)")    // description of i: Optional(5)
print("description of d: \(d as Double?)") // description of d: nil

Which can also be generalised to as Optional:

print("description of i: \(i as Optional)") // description of i: Optional(5)
print("description of d: \(d as Optional)") // description of d: nil

In Swift 5, with the new string interpolation system introduced by SE-0228, another option is to add a custom appendInterpolation overload for DefaultStringInterpolation:

extension DefaultStringInterpolation {
  mutating func appendInterpolation<T>(optional: T?) {
    appendInterpolation(String(describing: optional))
  }
}

var i: Int? = 5
var d: Double? = nil

print("description of i: \(optional: i)") // description of i: Optional(5)
print("description of d: \(optional: d)") // description of d: nil

And, if desired, you could even remove the argument label to disable the warning entirely within a module (or within a particular file if you mark it as fileprivate):

extension DefaultStringInterpolation {
  mutating func appendInterpolation<T>(_ optional: T?) {
    appendInterpolation(String(describing: optional))
  }
}

var i: Int? = 5
var d: Double? = nil

print("description of i: \(i)") // description of i: Optional(5)
print("description of d: \(d)") // description of d: nil

Though personally I would prefer to keep the argument label.