[discussion] What generics feature would this be?

Evolution Discussion
1

Curious what part of generics the below would fall under? This hit me by
surprise/makes even non-generic/Self-depending protocols un-interchangeable
with classes..

protocol Special {}

func doWithAndReturn<S: Special>(_ special: S) -> S { ... }

let special: Special = ...

// "error: Generics parameter 'S' could not be inferred"
// works fine when 'Special' is a class
let newSpecial: Special = doWithAndReturn(special)
2

A existential type of some protocol `P`, counterintuitively, doesn't conform to itself, unless it's an Objective-C protocol.

Because `S : Special` means S has to be a type that conforms to the protocol `Special`, and `special` is of the existential type `Special`, the existential type `Special` doesn't conform to the protocol `Special`, so it can't be used for `S`.

Best,
Austin

···

On Aug 9, 2017, at 7:19 AM, Mathew Huusko V via swift-evolution <swift-evolution@swift.org> wrote:

Curious what part of generics the below would fall under? This hit me by surprise/makes even non-generic/Self-depending protocols un-interchangeable with classes..

protocol Special {}

func doWithAndReturn<S: Special>(_ special: S) -> S { ... }

let special: Special = ...

// "error: Generics parameter 'S' could not be inferred"
// works fine when 'Special' is a class
let newSpecial: Special = doWithAndReturn(special)

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3

Very interesting. Is this likely to be changed in the future (either
existentials conforming to their protocols, or `S: Protocol` specifically
allowing for existentials)?

Also, is this at all related? Another instance of at naive/face value
something seeming like it should conform, but not..

protocol Special {}
protocol SubSpecial: Special {}

let spec: Special.Protocol = Special.self // works
let subspec: Special.Protocol = SubSpecial.self // doesnt work

···

On Wed, Aug 9, 2017 at 7:17 PM, Austin Zheng <austinzheng@gmail.com> wrote:

A existential type of some protocol `P`, counterintuitively, doesn't
conform to itself, unless it's an Objective-C protocol.

Because `S : Special` means S has to be a type that conforms to the
protocol `Special`, and `special` is of the existential type `Special`, the
existential type `Special` doesn't conform to the protocol `Special`, so it
can't be used for `S`.

Best,
Austin

> On Aug 9, 2017, at 7:19 AM, Mathew Huusko V via swift-evolution < > swift-evolution@swift.org> wrote:
>
>
> Curious what part of generics the below would fall under? This hit me by
surprise/makes even non-generic/Self-depending protocols un-interchangeable
with classes..
>
> ```
> protocol Special {}
>
> func doWithAndReturn<S: Special>(_ special: S) -> S { ... }
>
> let special: Special = ...
>
> // "error: Generics parameter 'S' could not be inferred"
> // works fine when 'Special' is a class
> let newSpecial: Special = doWithAndReturn(special)
> ```
> _______________________________________________
> swift-evolution mailing list
> swift-evolution@swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution