As we all know, if you override the default implementation of a protocol, if
you call the method from the protocol reference, the protocol’s default
implementation will be called, not the override of that method. For example:
protocol Fooable {
// this is intentionally blank
}
extension Fooable {
func foo() {
print("foo")
}
}
class FooBar: Fooable {
func foo() {
print("foobar")
}
}
func performFoo(object: Fooable) {
object.foo()
}
let foobar = FooBar()
performFoo(object: foobar) // prints “foo”, not “foobar"
But, the behavior changes you change the default method to be part of the
protocol, itself:
protocol Fooable {
func foo()
}
// the rest as above
performFoo(object: foobar) // prints “foobar”
I would like to propose that any class that overrides a protocol’s default
implementation where the method is not part of the protocol, itself, results in
a warning. I’m hard pressed to think of a situation where you’d want the
current Swift 3 behavior of the first example (where you can override a
protocol default implementation but you only want that override used when you
reference the class itself, but not when you reference the protocol as a type).
If there are such examples, then add a “build setting” to allow you to turn off
this warning, or add some keyword to the declaration of the default
implementation that indicates that you’re allowing it to be overridden, but
protocol types won’t use it (e.g. nondynamic). Personally, I’d just add the
warning and call it a day (because I don’t know why you’d ever want the current
Swift 3 behavior).
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