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

[Wagner Truppel via swift-users]

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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