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.

Two easier ways of dealing with this issue.

Option 1:

The first would be by "force-unwrapping" the value you would like to return using a bang (!)

var someValue: Int? = 5
print(someValue!)

Output:

5

Option 2:

The other way, which could be the better way - is to "safely-unwrap" the value you want returned.

var someValue: Int? = 5

if let newValue = someValue {
    print(newValue)
}

Output:

5

Would recommend to go with option 2.

Tip: Avoid force unwrapping (!) where possible as we are not sure if we will always have the value to be unwrapped.

seems using String(describing:optional) is simplest.

default value ?? makes no sense for non-Strings e.g Int.
If Int is nil then you want the log to show 'nil' not default to another Int e.g. 0.

Some playground code to test:

var optionalString : String? = nil
var optionalInt : Int? = nil

var description_ = ""
description_ = description_ + "optionalString: \(String(describing: optionalString))\r"
description_ = description_ + "   optionalInt: \(String(describing: optionalInt))\r"

print(description_)

Output

optionalString: nil
optionalInt: nil