I think the problem here is that P == P is true, but P : P is not (a protocol
does not conform to itself).
I think there was some discussion about it on the original "Completing
Generics" thread from March. I'd probably ask on the swift-users list why P
can't be made to conform to P, and then put together a proposal if there's no
good reason.
Austin
> On May 29, 2016, at 3:13 PM, Charles Srstka <cocoa...@charlessoft.com> wrote:
>
> Forgive me if this has already come up, but since we’re talking about fixing
> generics, I wonder if there is any solution in the pipeline for this problem:
>
> --
>
> protocol P { func foo() }
> struct S: P { func foo() { print("foo") } }
>
> func doSomething<C: CollectionType where C.Generator.Element: P>(c: C) {
> for each in c {
> each.foo()
> }
> }
>
> let arr: [P] = [S()]
>
> doSomething(arr) // error: cannot invoke 'doSomething' with an argument list
> of type '([P])’
>
> --
>
> Why is this an error? The whole definition of [P] is basically an array of
> things that conform to P. Isn’t that exactly what “where Element: P” is
> asking for?
>
> Changing Element: P to Element == P solves this particular issue, of course,
> but then passing something like [S] to the array will fail. The result is
> that you need to write two functions, and either have one eat the performance
> cost of constructing a new array that has the correct static type to pass to
> the other (since, unlike arrays, I can’t figure out a way to convert
> “Collection where Element: P” into “Collection where Element == P” with a
> simple cast), or just fill it with the dreaded copy-paste code. Neither seems
> ideal.
>
> Charles
>
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