Try using an associated type for the result of foo():
protocol P {
associatedtype R
static func foo<T>(_ v: T) -> R
}
Slava
> On Jul 7, 2017, at 1:50 AM, Jens Persson via swift-users
> <swift-users@swift.org> wrote:
>
> protocol P {
> // …
> // For example the following will not work:
> // static func foo<T, R>(_ v: T) -> R
> // Is there some other way?
> // …
> }
> struct S2 : P {
> static func foo<T>(_ v: T) -> (T, T) {
> return (v, v)
> }
> }
> struct S3 : P {
> static func foo<T>(_ v: T) -> (T, T, T) {
> return (v, v, v)
> }
> }
>
> (I'm guessing but am not sure that I've run into (yet) a(nother) situation
> which would require higher kinded types?)
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