[swift-users] Weird protocol behaviour.

[Kevin Nattinger]

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 <mailto: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 <http://balancingrock.nl/>
>> Blog: http://swiftrien.blogspot.com <http://swiftrien.blogspot.com/>
>> Github: http://github.com/Swiftrien <http://github.com/Swiftrien>
>> Project: http://swiftfire.nl <http://swiftfire.nl/>
>> 
>> 
>> 
>> 
>>> On 22 Dec 2016, at 17:05, Mikhail Seriukov via swift-users <swift-users at swift.org <mailto: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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