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

[Jacob Bandes-Storch]

The same ambiguity occurs even without inheritance:

class C {
    func foo() {}
    func foo(x: Int = 0) {}
}

Somewhat related: https://bugs.swift.org/browse/SR-1408


On Wed, Jan 4, 2017 at 7:42 PM, Wagner Truppel via swift-users <
swift-users at swift.org> wrote:

> I’m afraid I wasn’t clear enough on my post. The default value I referred
> to is the “= 0”. Had it been absent, the call c.foo() would undeniably be
> fulfilled by the parent class. However, with the default argument value,
> it’s not obvious whether c.foo() should be the parent’s implementation or
> the child’s implementation (with the argument set to the default value).
> That’s why I’m suggesting a compiler warning because it’s very easy to make
> the mistake of calling c.foo() expecting the child’s implementation and it
> may be a hard bug to track when it happens.
>
> Wagner
>
>
> On 5 Jan 2017, at 03:35, Saagar Jha <saagar at saagarjha.com> wrote:
>
> I’m not quite sure what you mean by "restrictions of parent
> implementations”, however, the “default value” you’re mentioning is a
> fundamental part of OOP–when a subclass overrides a superclass, it gets the
> parent class’s methods for free. There’s no need to issue a warning for
> this, since it’s expected behavior from other Object-Oriented languages.
>
> Saagar Jha
>
>
>
> On Jan 4, 2017, at 6:29 PM, Wagner Truppel via swift-users <
> swift-users at swift.org> wrote:
>
> Hello,
>
> 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.
> Cheers,
>
> Wagner
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>
>
>
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