When a class conforms to a protocol and a requirement is fulfilled by a method
in an extension, the class does not get a vtable entry for the extension
method. So it cannot be overridden in a subclass — there’s nothing to
dynamically dispatch here. We plan on addressing this as part of ABI stability.
Slava
> On Feb 22, 2017, at 9:39 AM, David Hart via swift-users
> <swift-users@swift.org> wrote:
>
> In the following piece of code, can somebody explain the last result? Why the
> break in consistency? Is this a bug?
>
> protocol P {
> func foo() -> String
> }
>
> extension P {
> func foo() -> String { return "P" }
> }
>
> class A : P {
> func foo() -> String { return "A" }
> }
>
> class B : P {}
> class C : B {
> func foo() -> String { return "C" }
> }
>
> A().foo() // A
> (A() as P).foo() // A
> B().foo() // P
> (B() as P).foo() // P
> C().foo() // C
> (C() as P).foo() // P
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