Swift extension with a default parameter - 🐛 bug

Example.swift

protocol Foo {
    func foo(_ int: Int)
}

extension Foo {
    func foo(_ int: Int = 1) { print("foo") }
    func bar() { print("bar") }
}

func testFoo(foo: Foo) {
    foo.foo()
    foo.bar()
}

struct FooImpl: Foo {
    
}

struct FooStub: Foo {
    func foo(_ int: Int) { print("stubbed foo") }
    func bar() { print("stubbed bar") }
}

testFoo(foo: FooImpl())
testFoo(foo: FooStub())

OverloadExample.swift

protocol Foo {
	func foo(_ int: Int)
}

extension Foo {
	func foo() { self.foo(1) }
	func foo(_ int: Int) { print("foo (\(int))") }
	func bar() { print("bar") }
}

func testFoo(foo: Foo) {
	foo.foo()
	foo.bar()
}

struct FooImpl: Foo {
	
}

struct FooStub: Foo {
	func foo(_ int: Int) { print("stubbed foo (\(int))") }
	func bar() { print("stubbed bar") }
}

testFoo(foo: FooImpl())
testFoo(foo: FooStub())

That's interesting @olliatkinson! It definitely is confusing, but (i think) it's operating as intended. I believe there is some thoughts to add default values to protocols so this behavior becomes clearer. Your OverloadExample is the best way to obtain this functionality at this point I believe.

What is happening is testFoo is using direct dispatch to the extension method with the default parameter. The protocol has no way of knowing else to do, so the method in FooStub is never called. If the protocol could state that it needs a default value, the behavior would be more clear, but for now it's not.

I did a quick scan of https://bugs.swift.org/ and didn't see anything, but log a bug there if you want an official opinion.