[swift-users] Weird protocol behaviour.

[Mikhail Seriukov]

Ok,
But I think I still do not get it.
What does really happen when we write this?

let x = X() as P

As I said, I expect x to be Any<P> after that. If it is, then it should be
ok IMO.
But if it is not then what is the actual type of x?

So the real question is how the type checker works here?


2016-12-25 22:13 GMT+07:00 Slava Pestov <spestov at apple.com>:

>
> On Dec 22, 2016, at 4:43 PM, Howard Lovatt via swift-users <
> swift-users at swift.org> wrote:
>
> The following variation works:
>
> protocol P {}
>
> class P1:P {}
>
> class X:P1 {}
>
> func foo<A:P>(_ x:A) {}
>
> func bar() {
>     //let x = X() // this compiles
>     let x = X() as P1 // this does not compile. Why?
>     foo(x)
> }
>
> Which adds credence to the bug theory.
>
>
> It’s an intentional limitation. Protocols do not conform to themselves.
> Lifting the restriction would be difficult to do efficiently given our
> representation of generics and protocols at runtime.
>
> Slava
>
>
> Note two changes: 1. two levels of inheritance and 2. change to classes.
> If you do two levels using protocols it doesn't work if you use either
> classes or structs.
>
>
>   -- Howard.
>
> On 23 December 2016 at 07:29, Kevin Nattinger <swift at nattinger.net> wrote:
>
>> I recall seeing a request on the -evolution list for something like `T :=
>> X` to indicate it could be X itself or anything inheriting / implementing
>> it, so it’s certainly known behavior, if not desired. IMO it’s a bug and
>> `:` should be fixed to include the root type, whether or not that requires
>> a discussion on -evolution.
>>
>> On Dec 22, 2016, at 2:17 PM, Howard Lovatt via swift-users <
>> swift-users at swift.org> wrote:
>>
>> I suspect a compiler bug since A is a P. The equivalent in Java works:
>>
>> interface P {}
>> class X implements P {}
>>
>> <A extends P> void foo(A x) {}
>>
>> void bar() {
>>     final P x = new X();
>>     foo(x);
>> }
>>
>> -- Howard.
>>
>> On 23 Dec 2016, at 3:19 am, Rien via swift-users <swift-users at swift.org>
>> wrote:
>>
>> IMO the error message says it all:
>>
>> Playground execution failed: error: MyPlayground8.playground:9:5: error:
>> cannot invoke 'foo' with an argument list of type '(P)'
>>    foo(x)
>>    ^
>>
>> MyPlayground8.playground:9:5: note: expected an argument list of type
>> '(A)'
>>    foo(x)
>>    ^
>>
>> I.e. you are passing in a protocol while the function is specified for a
>> type.
>> Said other way: On which data do you expect the protocol to operate?
>>
>> Regards,
>> Rien
>>
>> Site: http://balancingrock.nl
>> Blog: http://swiftrien.blogspot.com
>> Github: http://github.com/Swiftrien
>> Project: http://swiftfire.nl
>>
>>
>>
>>
>> On 22 Dec 2016, at 17:05, Mikhail Seriukov via swift-users <
>> swift-users at swift.org> wrote:
>>
>>
>> Hello community! I' wondering if somebody can explain this to me.
>>
>> Please take look at the snippet.
>>
>>
>> protocol P {}
>>
>> struct X:P {}
>>
>>
>> func foo<A:P>(_ x:A) {}
>>
>>
>> func bar() {
>>
>>    //let x = X() // this compiles
>>
>>    let x = X() as P // this does not compile. Why?
>>
>>    foo(x)
>>
>> }
>>
>>
>> I expect the both cases to work though. But only first works? And I do
>> not understand why.
>>
>> My coworkers said that it is a compiler bug, but I'm not shure it is.
>>
>> Thanks for the help.
>>
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>>
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>>
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