That was fixed in Swift 3.0 following the acceptation of SE-0046.
In Swift 1.0, this applied to methods but not functions as it was following up from Objectice-C (foo:bar:baz: would become foo(_:bar:baz)), in Swift 2.0 it was expanded to all callables, and in Swift 3.0 it was removed and to be specified explicitly making labelling completely consistent.
In Swift 1.0, this applied to methods but not functions as it was following up from Objectice-C (foo:bar:baz: would become foo(_:bar:baz)), in Swift 2.0 it was expanded to all callables, and in Swift 3.0 it was removed and to be specified explicitly making labelling completely consistent.