No, inout should *not* be covariant, since foo can assign any subtype of P to
its parameter, not just F:
protocol P {}
struct F: P {}
struct G: P {}
func foo(_ p: inout P) {
p = G()
}
var f = F()
foo(&f) // assigns a value of type G to a variable of type F
>From Jonathan
> On Dec 10, 2017, at 12:51 AM, Adrian Zubarev via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> Hello Matthew, I have more more question about the generalized supertype
> constraints. Does the generalization also covers inout? I found a really
> annoying case with inout.
>
> protocol P {}
>
> func foo(_ p: inout P) {
> print(p)
> }
>
> struct F : P {}
>
> var f = F()
>
> foo(&f) // won't compile until we explicitly write `var f: P = F()`
> Shouldn’t we allow inout to have subtypes as well, like inout F : inout P?
>
> This is actually one of the pain points with the generic version of the print
> function. We cannot pass an arbitrary TextOutputStream to it without
> sacrificing the type. I doubt the generic print function is justified,
> because TextOuputStream does not have associated types nor a Self constraint.
> Furthermore it forces you to create a custom type-erased wrapper that can
> hold an arbitrary TextOutputSteram.
>
> If this is part of a totally different topic, I’ll move it in it’s own thread.
>
>
>
> Am 2. Dezember 2017 um 19:03:24, Matthew Johnson via swift-evolution
> (swift-evolution@swift.org) schrieb:
>
>> This thread received very light, but positive feedback. I would really like
>> to see this feature added and am willing to draft and official proposal but
>> am not able to implement it. If anyone is interested in collaborating just
>> let me know.
>>
>>
>>> On Nov 24, 2017, at 5:03 PM, Matthew Johnson via swift-evolution
>>> <swift-evolution@swift.org> wrote:
>>>
>>> One of the most frequent frustrations I encounter when writing generic code
>>> in Swift is the requirement that supertype constraints be concrete. When I
>>> mentioned this on Twitter
>>> (https://twitter.com/anandabits/status/929958479598534656) Doug Gregor
>>> mentioned that this feature is smaller and mostly straightforward to design
>>> and implement (https://twitter.com/dgregor79/status/929975472779288576).
>>>
>>> I currently have a PR open to add the high-level description of this
>>> feature found below to the generics manifesto
>>> (https://github.com/apple/swift/pull/13012):
>>>
>>> Currently, supertype constraints may only be specified using a concrete
>>> class or protocol type. This prevents us from abstracting over the
>>> supertype.
>>>
>>> ```swift
>>> protocol P {
>>> associatedtype Base
>>> associatedtype Derived: Base
>>> }
>>> ```
>>>
>>> In the above example `Base` may be any type. `Derived` may be the same as
>>> `Base` or may be _any_ subtype of `Base`. All subtype relationships
>>> supported by Swift should be supported in this context including, but not
>>> limited to, classes and subclasses, existentials and conforming concrete
>>> types or refining existentials, `T?` and `T`, `((Base) -> Void)` and
>>> `((Derived) -> Void)`, etc.
>>>
>>> Generalized supertype constraints would be accepted in all syntactic
>>> locations where generic constraints are accepted.
>>>
>>> I would like to see generalized supertype constraints make it into Swift 5
>>> if possible. I am not an implementer so I will not be able to bring a
>>> proposal forward alone but am interested in collaborating with anyone
>>> interested in working on implementation.
>>>
>>> I am also interested in hearing general feedback on this feature from the
>>> community at large. Have you also found this limitation frustrating? In
>>> what contexts? Does anyone have reservations about introducing this
>>> capability? If so, what are they?
>>>
>>> Matthew
>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution@swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
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