Xcode 8 / Swift 3: "Expression of type UIViewController? is unused" warning

TL;DR

popViewController(animated:) returns UIViewController?, and the compiler is giving that warning since you aren't capturing the value. The solution is to assign it to an underscore:

_ = navigationController?.popViewController(animated: true)

Swift 3 Change

Before Swift 3, all methods had a "discardable result" by default. No warning would occur when you did not capture what the method returned.

In order to tell the compiler that the result should be captured, you had to add @warn_unused_result before the method declaration. It would be used for methods that have a mutable form (ex. sort and sortInPlace). You would add @warn_unused_result(mutable_variant="mutableMethodHere") to tell the compiler of it.

However, with Swift 3, the behavior is flipped. All methods now warn that the return value is not captured. If you want to tell the compiler that the warning isn't necessary, you add @discardableResult before the method declaration.

If you don't want to use the return value, you have to explicitly tell the compiler by assigning it to an underscore:

_ = someMethodThatReturnsSomething()

Motivation for adding this to Swift 3:

• Prevention of possible bugs (ex. using sort thinking it modifies the collection)
• Explicit intent of not capturing or needing to capture the result for other collaborators

The UIKit API appears to be behind on this, not adding @discardableResult for the perfectly normal (if not more common) use of popViewController(animated:) without capturing the return value.

Read More

• SE-0047 Swift Evolution Proposal
• Accepted proposal with revisions

When life gives you lemons, make an extension:

import UIKit

extension UINavigationController {
    func pop(animated: Bool) {
        _ = self.popViewController(animated: animated)
    }

    func popToRoot(animated: Bool) {
        _ = self.popToRootViewController(animated: animated)
    }
}

Note that adding something like @discardableResult func pop(animated: Bool) -> UIViewController? will result in the same warning you are trying to avoid.

With the extension you can now write:

func exitViewController()
{
    navigationController?.pop(animated: true)
}

func popToTheRootOfNav() {
    navigationController?.popToRoot(animated: true)
}

Edit: Added popToRoot too.

In Swift 3, ignoring the return value of a function that has a declared return value results in a warning.

One way to opt out of this is to mark the function with the @discardableResult attribute. Since you don't have control over this function, that won't work.

The other method to get rid of the warning is to assign the value to _. This tells the compiler you know the method returns a value but you don't want to retain it in memory.

let _ = navigationController?.popViewController(animated: true)

Although it work correctly if kept as it is but the number of warning increases.

The solution is to simply replace it with underscore ( _ ) though it seems to be ugly.

Eg.  _ = navigationController?.popViewController(animated: true)

Use discardableResult in this condition.

According to < Swift Programming Language > , chapter Language Reference - Attributes.

discardableResult

Apply this attribute to a function or method declaration to suppress the compiler warning when the function or method that returns a value is called without using its result.

There is also a demo in < Swift Programming Language >, chapter Language Guide - Methods.

@discardableResult
mutating func advance(to level: Int) -> Bool {
    ...
    return true
}

Because it’s not necessarily a mistake for code that calls the advance(to:) method to ignore the return value, this function is marked with the @discardableResult attribute. For more information about this attribute, see Attributes.