Why does compiler warn of unused result on function marked @discardableResult?

Question

The Swift 3 compiler is warning me of an unused result, even though I have marked the function with @discardableResult.

It is only occurring when calling the function on an optional variable, using the ? syntax.

To simplify the problem, I created this sample code. (I had to put it in a project because the warning didn't show in a playground.)

class Foo {
    @discardableResult func bar() -> String? {
        return "bar"
    }
}

class Tester {
    func doSomething() {
        var foo: Foo?

        foo = Foo()

        foo?.bar() //Warning: Expression of type 'String?' is unused
        foo!.bar() //No warning
    }
}

Answer 1

It's a known bug (https://bugs.swift.org/browse/SR-1681), though the Swift gang seems to think it has been resolved. Maybe the fix hasn't made it into Xcode yet, or maybe they're just wrong.

UPDATE We got the official word: The fix will appear starting in Xcode 8.3.