[swift-users] Weird function call behaviour

[David Hart]

> On 22 Feb 2017, at 22:02, Slava Pestov via swift-users <swift-users at swift.org> wrote:
> 
> 
>> On Feb 22, 2017, at 1:01 PM, David Hart <david.w.hart at me.com <mailto:david.w.hart at me.com>> wrote:
>> 
>> 
>>> On 22 Feb 2017, at 21:59, Slava Pestov via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>> 
>>> 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.
>> 
>> What do you mean by “address” this? Force the vtable entry?
> 
> Yeah. Instead of having the witness table for the conformance point directly to the extension method, if the conforming type is a class, we will generate a small thunk that calls a vtable entry, and store the extension method in that vtable entry.
> 
> Of course this will only work if the conformance is defined in the same translation unit as the class; if you define the extension in a different module, you’ll get the same behavior you observe below.

Sound great :)

> Slava
> 
>> 
>>> Slava
>>> 
>>> 
>>>> On Feb 22, 2017, at 9:39 AM, David Hart via swift-users <swift-users at swift.org <mailto:swift-users at 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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>>> 
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> 
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