[swift-users] Weird protocol behaviour.

Rien Rien at Balancingrock.nl
Thu Dec 22 10:19:27 CST 2016

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)'

MyPlayground8.playground:9:5: note: expected an argument list of type '(A)'

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?


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> 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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