[swift-users] Compiler should issue a warning when a subclass implementation with default values matches a parent implementation without them

Wagner Truppel trupwl at gmail.com
Wed Jan 4 20:29:57 CST 2017


I wasn’t sure whether to post this message here, at swift-dev, or at swift-evolution. so I’ll try here first. Hopefully it will get to the right group of people or, if not, someone will point me to the right mailing list.

I came across a situation that boils down to this example:

class Parent {
    func foo() {
        print("Parent foo() called")

class Child: Parent {
    func foo(x: Int = 0) {
        print("Child foo() called")

let c = Child()
c.foo()  // prints "Parent foo() called"

I understand why this behaves like so, namely, the subclass has a method foo(x:) but no direct implementation of foo() so the parent’s implementation is invoked rather than the child's. That’s all fine except that it is not very intuitive.

I would argue that the expectation is that the search for an implementation should start with the subclass (which is does) but should look at all possible restrictions of parent implementations, including the restriction due to default values.

At the very least, I think the compiler should emit a warning or possibly even an error.

Thanks for reading.


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